Considering the sins as Major or Minor:

Considering the sins as Major or Minor:

According to the ahadeeth, sins are broadly classified as Major (kabeera) or Minor (sagheera) sins. Their numbers vary according to various accounts and narrations.
Generally, it is held that any sin for which a punishment in the dunya or aakhirat has been defined in the Quran or hadith is a major sin.

However this differentiation is not always present.
The scholars say that any minor sin that is committed habitually is counted as a major sin.

Given below are some interesting insights of the salaf on this issue:

Qadhi ‘Iyad رحمه الله  said:
“According to how small you consider  the sin, that much big will it be in the sight of Allah, and according to how great you consider the sin that much smaller will it be in the sight of Allah.” [As-Siyar, 8/428]

Imam Al- Awza’i رحمه الله  said:
“Once I heard Bilal Ibn Sa’d saying, ‘Do not look at how small the sin is but instead look at how great is the One that you have disobeyed.’ “ [Al-Hilya, 5/223]

Abu Hazim Salamah Ibn Dinar said:
“Look at every act that you would hate to die while committing, and then abandon it.” [Al-Musannaf, 7/194]

Ibn ‘Abbas رضي الله ﺗﻌﺎﻟﯽٰ عنه said:
“There cannot be a major sin along with seeking Allah's forgiveness, nor a minor sin while being persistent upon it.”
[Ash-Shu’ab, 5/7268]

Imam Al- Awza’i رحمه الله  said:
“From the major sins is for a man to do a sin and then look down upon it (considering it as minor or inconsequential)." 
[Ash-Shu’ab, p.6752]

The 3 conditions of a successful servant of Allah- Ibn Qayyim

Three conditions are tokens of the servant’s happiness [sa’adat al-abd], and the signs of his success in this world and the next. No servant is without them, but is always shifting from one to the other. These are: 1. When blessed, give thanks (Shukr); 2. when tried with difficulties/trials, persevere (Sabr); and 3. when sinful, seek forgiveness (Tawba).              

1. Shukr for blessings:
The first condition is the blessings which come to the servant from God (Most High), one after another. What secures them is gratitude [shukr], based on three supports: (a) inward recognition of the blessing; (b) outward mention and thanks for it; (c) and its use in a way that pleases the One to whom it truly belongs and who truly bestows it. Acting thus, the servant shows his gratitude for the blessing—however brief.

2. Sabr in trials:
The second is the trials from God (Most High) which test the Servant, whose duty therein is patience [sabr] and forbearance: (a) to restrain himself from anger with what is decreed; (b) to restrain his tongue from complaint; (c) to restrain his limbs from offences, such as striking one’s face in grief, rending one’s clothes, tearing one’s hair and like acts. Patience, then, rests on these three supports, and if the servant maintains them as he should, affliction will become benefaction, trial will change to bounty and what he disliked will become what he loves. For God (Exalted and Sublime) does not try the servant in order to destroy him. Rather, He tries him to put his patience and devotion [al-ubudiyya] to the test. For the servant owes devotion to God in affliction as in ease. He must have as much devotion in what he hates as in what he loves. And while most people offer devotion in what they love, it is important to do so in the things they hate. It is by this that servants’ ranks are distinguished and their stations determined.

Ablution with cold water in searing heat is devotion. Sexual relations with one’s beautiful and beloved spouse is devotion. Spending money for her, for one’s children and for oneself is devotion. It is devotion no less than ablution with cold water in the bitter cold; giving up vice to which one’s soul is driven without fear of people; and giving charity in hardship. But there is a great difference between the [two kinds] of devotion.

He who is God’s servant in both states, maintaining his duty in both comfort and adversity, is the one to whom His words refer, ‘Is not God sufficient for His servant?’ With complete devotion comes complete sufficiency, and with less comes what is less. Let him who discerns some good give praise to God, but let whoever finds something other than this blame no one but himself.

These are the servants over whom God’s Foe has no control. God said [to the Devil], ‘Lo! As for My servants, you have no power over them.” And when His Foe Iblis learned that He would not let His devoted servants yield to him or give him control over them, he proclaimed, ‘Then by Your Might, I will surely beguile them all save for Your sincere servants among them. And God (Most High) said, ‘And Iblis found his calculation true, for they [all] followed him save a group of true believers. And he had no warrant whatsoever over [any of] them save that We might know the ones who believe in the hereafter from those who doubt it.’ God will not yield to His Foe control over His faithful servants. They are in His protection and His care. If the Devil robs any of them, as the thief robs the heedless man, this cannot be avoided, because by heedlessness, passion and anger is the servant tried. It is by these same three doors that the Devil comes to him. Try as he may to protect himself, the servant is bound to be heedless and given to passion and anger.

3.Tawba after sinning:
Adam, the father of all humanity, was the most discerning of creatures, their superior in wisdom, and the most steadfast. Yet the Foe kept after him until he made him fall into that which he fell. What then of someone with the reason of a moth, whose intelligence compared to that of his father [Adam] is like a spittle in the ocean? Still, the Foe of God obtains nothing from a faithful person except by robbing him in [a moment on inattention and carelessness. And when he causes him to fall, the servant may think that he can never again face his Lord, that this fall has carried him away and destroyed him. Yet behind it all is God’s grace, mercy, clemency and forgiveness.

For if God intends what is good for His servant, He will then open for him the doors of repentance [al-tawba] and remorse, abasement and humility, dependence and need; the doors of the request for God’s help and protection; the doors of perpetual humility, supplication and the approach towards Him by means of whatever good works he can manage—so that his wrong may become a means to God’s mercy. For the Foe says, ‘Alas, I left him without causing him to fall!’

This is what one of the early believers [salaf] meant when he said, ‘A person may commit a sin by which he goes to heaven and a good deed by which he goes to hell.’ ‘How?’ someone asked. He replied, ‘Having committed the sin, he is ever watchful in fear, regretful, timorous, lamenting, shamed before his Lord, his head in his hands and his heart rent. The sin that brings him all that we have mentioned, wherein lie his happiness and salvation, is more beneficial to him than numerous devotional acts. Indeed, it becomes the means by which he enters Heaven.

[On the other hand], he may perform a goodly deed and constantly laud it before his Lord, wax proud, boast, become vain and haughty with it, as he says, ‘I did this, I did that.’ His self-importance, pride and arrogance provide him only with the means to his own ruin. If God intends then what is good for this miserable person, He will try him through something that breaks [his pride], abases him and reduces his self-importance. But if He intends otherwise, He will leave him to his self-importance and pride, and this misfortune is what leads to his ruin.’

-From Ibn Qayyim al-Jawziyya's 'The Invocation of God' (Al-Wabil al-Sayyib)

On asking forgiveness - 'Riyadus Saliheen'

On asking forgiveness - From 'Riyadus Saliheen'

by Imam Abu Zakaria Mohiuddin Yahya An-Nawawi (RA):

 Translation by Ustadha Aisha Bewley

From the Holy Quran-

Allah says, "Ask forgiveness for your wrongdoing, and for the men and women who believe," (47:20; 47:19)

 and the Almighty says, "Ask Allah's forgiveness. Allah is Ever-Forgiving, Most Merciful." (4:105; 4:106)

 He says, "Then glorify your Lord's praise and ask His forgiveness. He is the Ever-Returning." (110:3-4; 110:3)

and the Almighty says, "Those who are God-fearing will have Gardens with their Lord with rivers flowing under them" to His words, "those who seek forgiveness before dawn." (3:15-17)

Allah says, "Anyone who does evil or wrongs himself and then asks Allah¹s forgiveness will find Allah Ever-Forgiving, Most Merciful." (4:109; 4:110)

and He says, "Allah would not punish them while you were among them. Allah would not punish them as long as they sought forgiveness." (8:33)

 The Almighty says, "Those who, when they act indecently or wrong themselves, remember Allah and ask forgiveness for their bad actions – and who can forgive bad actions except Allah – and do not knowingly persist in what they were doing." (3:135)

From the Hadith-

1869. Al-Agharr al-Muzani reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "A cloudiness comes over my heart and I ask for Allah's forgiveness a hundred times a day." [Muslim]

 1870. Abu Hurayra said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, say, 'By Allah, I ask Allah's forgiveness and turn to Him more than seventy times a day.'" [al-Bukhari]

 1871. Abu Hurayra reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "By the One in whose hand my soul is, if you did not do wrong, Allah Almighty would remove you and bring a people who do wrong and then ask Allah Almighty for forgiveness and He would forgive them." [Muslim]

1872. Ibn 'Umar said, "We used to count out for the Messengerof Allah, may Allah bless him and grant him peace, in one assembly a hundred times, "Lord forgive me and turn to Me, You are the Returning, the Merciful." [Abu Dawud and at-Tirmidhi]

  1873. Ibn 'Abbas reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "If someone clings to asking for forgiveness, Allah will appoint him a way out of every constriction, and relief from every care and will provide for him from where he does not reckon."[Abu Dawud]

1874. Ibn Mas'ud reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "Anyone who says, 'I ask forgiveness of Allah, other than whom there is no god, the Living, the All-Sustaining and I turn to him,' will be forgiven his wrong actions, even if he fled from the press of battle." [Abu Dawud and at-Tirmidhi]

1875. Shaddad ibn Aws stated that the Prophet, may Allah bless him and grant him peace, said, "The best way to ask forgiveness is for a slave to say, 'O Allah, You are my Lord. There is no god but You. You created me and I am Your slave. I comply with Your covenant and Your promise as much as I can. I seek refuge with you from the evil of what I have done. I acknowledge my sin, so forgive me. Only You can forgive sins.' Anyone who says this during the day having confidence in it and dies on that day before evening will be among the people of the Garden. Anyone who says it during the night having confidence in it and dies before morning will be among the people of the Garden." [al-Bukhari]

 1876. Thawban said, "When the Messenger of Allah, may Allah bless him and grant him peace, used to finish his prayer, he asked forgiveness three times and said, 'O Allah, You are Peace and peace is from You. You are blessed, O Possessor of majesty and nobility.'" Al-Awza'i, one of the transmitters of the hadith, was asked, "How was the asking forgiveness?" He said, "You say, 'I ask Allah's forgiveness. I ask Allah's forgiveness.'". [Muslim]

1877. 'A'isha said, "The Messenger of Allah, may Allah bless him and grant him peace, used to often say before his death, 'Glory be to Allah and by His praise. I ask Allah's forgiveness and I turn in repentance to Him." [Agreed upon]

 1878. Anas said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, say, 'Allah Almighty says, "O son of Adam! I will forgive you as long as you call on Me and have hope in Me, no matter what you do. Son of Adam, I do not care if your wrong actions reach to the clouds of heaven and then you ask Me for forgiveness, I will forgive you. Son of Adam, if you were to come with sins equivalent in weight to the whole earth and then meet Me having not associated anything with Me, I would come to you with the same amount of forgiveness.'" [at-Tirmidhi]

 1879. Ibn 'Umar stated that the Prophet, may Allah bless him and grant him peace, said, "O company of women! Give sadaqa and ask forgiveness often. I have seen that you comprise the majority of the people of the Fire." One of the women said, "Why are we are the majority of the people of the Fire?" He said, "You curse a lot and you are ungrateful to your husbands. Despite the fact that I see you are deficient both in intellect and the deen, intelligent men are destroyed by you." She said, "What is our deficiency in intelligence and the deen?" He said, "The testimony of two women is only worth the testimony of one man and there are days when a woman does not pray." [Muslim]