"Say: O Ibadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful" (Quran 39:53)
Whether it's watching porn, lying, backbiting or other vices, our sins accumulate daily.
We may try to hold our tongues, lower our gaze, not disrespect our parents or avoid a myriad of sins God has ordered us to avoid, but as humans we are weak, and we can and do make mistakes.
But while God is just and holds us responsible for our actions, the doors to His forgiveness are always open; doing Haram and seeking forgiveness in Islam is always there. No matter how awful our sin, Allah is always ready to hear and accept our repentance, provided it is sincere and results in change for the better, so never stop making dua for forgiveness.
In the Quran, God repeatedly reminds us that He is the Forgiving and the Merciful.
Read Quranic verses from sura Al-Furqan (Quran 25: 63-73) for a better understanding of these issue. Here is a translation of some Quranic verses and its commentary (Quran 25: 68- 70):
"Who invoke no other deity along with Allah, nor take any life which Allah has forbidden save justly; who do not commit unlawful sexual intercourse and whoso does that shall meet its penalty;
His torment shall be doubled for him on the Day of Resurrection, and he will abide in it in ignominy
Unless he repents and reaffirm his believes and does righteous works.(1) For such, Allah will change their evil deeds to good deeds.(2) Allah is Ever Forgiving, Most Compassionate."
(1). This is good news for those whose life has been stained with various kinds of sin in the past but who later repent and firmly decide to amend their behavior. The present verse amounts to a proclamation of general amnesty, which for millions of sinners provided a ray of hope and prompted them to reform themselves.
The extent to which this provision of repentance saved the Arabs who were otherwise given to many a vice is evident from a number of incidents during the life of the Prophet (Peace be upon him). Let us consider the following example, narrated by Tabari on the authority of Abu Hurayrah who said: "One night when I returned from the Prophet's Mosque after offering Isha Prayers, I saw a woman standing outside my door. I said 'Peace be on you' and went into my room, closed the door, and began to offer nafl (supererogatory) Prayers. After some time, she knocked at the door. I opened the door and asked her what she wanted. 'I have come to ask you a question. I committed adultery and became pregnant. Then a baby was born and I killed him. Now I have come to ask you if there is any chance that my sin be forgiven?' I said: 'Absolutely not'. At this she went away grief-stricken, exclaiming: 'Alas, this piece of beauty was created for the Hell-fire.'
"The next day, after the Fajr prayers, I narrated the whole incident to the Prophet (peace be on him). He said: 'How wrong an answer you gave, O Abu Hurayrah! Have you not read those verses in the Quran: "Those who invoke no other deity along with Allah nor take the life which Allah has forbidden – save justly; who do not commit unlawful sexual intercourse – and whoso does that shall meet its penalty, his torment shall be doubled for him on the Day of Resurrection and he shall abide in it in ignominy – unless he who repents and believes and does righteous works. For such Allah will change their evil deeds to good deeds. Allah is Ever Forgiving, Most Compassionate. Whosoever repents and does good, he returns to Allah in the manner he should" (Al-Furqan 25:68-70). Upon hearing this answer from the Prophet (Peace be on him), I went out and searched for the woman. I met her after Isha Prayers and told her what the Prophet (peace be on him) had said. She prostrated herself and said, 'Thanks be to God, Who has opened the door of forgiveness for me.' Then she repented and freed a female slave and also the son of that slave." (See Tabari, 'Tafsir', comments on Surah al-Furqan 25:68-70 Ed.)
(2). Doing Haram and seeking forgiveness in Islam, this has two meanings. One, that when a person repents and gives up his evil actions and begins to perform vitreous deed, in this new phase of life, of Faith and righteous conduct, his good deeds will replace the evil deeds of his past. Second, that as a result of his repentance, not only are the evil deeds which he committed during this period of disbelief and sinful living forgiven, but each person will be awarded for having given up a life of disbelief and transgression and for replacing it with obedience and Faith. Moreover, for as many times as such people feel ashamed of their past evil deeds, they will be rewarded that same number of times for having asked for forgiveness from God. Feeling ashamed of one's sins and asking for forgiveness is, in itself, a good deed. In this manner, all of one's evil deeds will eventually be replaced by good deeds and one will not only be saved from punishment, but will also be amply rewarded.
Commentary from Towards Understanding the Quran with some abbreviation