First, sin should not be something that leads us to despair, for despair and guilt has the danger to lead to more sin, which in turn leads to more guilt, and it becomes a vicious cycle. Despair is Satan’s trick to make us fall back into sin. The Quran says:
قُلۡ یٰعِبَادِیَ الَّذِیۡنَ اَسۡرَفُوۡا عَلٰۤی اَنۡفُسِہِمۡ لَا تَقۡنَطُوۡا مِنۡ رَّحۡمَۃِ اللّٰہِ ؕ اِنَّ اللّٰہَ یَغۡفِرُ الذُّنُوۡبَ جَمِیۡعًا ؕ اِنَّہٗ ہُوَ الۡغَفُوۡرُ الرَّحِیۡمُ ﴿۵۴﴾
“Say, ‘O My servants who have committed excesses against their own souls! Despair not of the mercy of Allah, surely Allah forgives all sins. Verily He is Most Forgiving, Merciful’” (39:54).
The Holy Prophet(sa) once narrated the story of a person who murdered 99 people and then felt remorse. He asked for the most knowledgeable person around and was told about someone. When he asked him if he had any chance at repentance and salvation, the other man expressed shock, and the murderer took out his sword and killed this man too and now the number of murders stood at 100. Then he searched for another knowledgeable person and he was told about another person. When he went to him and asked him about the possibility of repentance, he was told, “Leave the evil town where you are living and go to a good town”. The murderer decided to do this but he died while on the way. Then, according to the hadith, the angels of mercy and punishment argued over him. Eventually, God sent an angel who made the final judgment in the words, “Look and see which of the two towns he was closer to, and put him with its people”. Since he was closer to the town of good people, he was forgiven.[i]
This story has a clear, take-away moral message, which is, the door to repentance is always open till a person’s death. In another hadith, the Holy Prophet(sa) said:
اَلتَّائِبُ مِنَ الذَّنْبِ كَمَنْ لاَ ذَنْبَ لَهُ
That is, “The one who repents from sin is like one who did not sin”.[ii] The Quran calls God Ghafoor (The One Who forgives), and states:
قَالَ عَذَابِیۡۤ اُصِیۡبُ بِہٖ مَنۡ اَشَآءُ ۚ وَ رَحۡمَتِیۡ وَسِعَتۡ کُلَّ شَیۡءٍ ؕ
That is, “God said, ‘I will inflict My punishment on whom I will; but My mercy encompasses all things’” (Holy Quran, 7:157). God has given us great hope in this verse and clearly declared that His mercy encompasses everything else. No sin is beyond the mercy of Allah as long as there is sincere repentance prior to a person’s death.
When a person has committed a sin, the important thing to do is to keep on reciting Istighfar, in the words:
اَسْتَغْفِرُ اللهَ رَبِّيْ مِنْ كُلِّ ذَنْبٍ وَّ اَتُوْبُ اِلَیْه
That is, “I seek forgiveness from Allah for all my sins and turn towards Him in all sincerity”. It should be kept in mind here that some people have the habit of reciting the Arabic alone without any understanding of the meaning, and it is presumed that they have done Istighfar. However, the Promised Messiah(as) has taught us that Istighfar should come from the heart and can be done in a person’s own language, if it is difficult to keep track of the meaning of the Arabic words. The Holy Prophet(sa) used to recite Istighfar 70-100 times daily. We should be following his example, especially at a time when a sin has been committed.
Second, we should try to do good deeds to remove the aftereffects of evil. The Quran says, “Good works drive away evil works” (Holy Quran, 11:115). Some of the good works include offering prayers in congregation, offering nawaafil with special prayers in the sajdah in our own language seeking Allah’s mercy, giving some sadaqah, doing various forms of zikr, doing Jama‘at work, and so on.
Third, we should try to find good company. Seeking out friends who are positive and motivate us to do good works is a great asset in self-reformation. Congregational prayer at the mosque or Namaz centre, for instance, can help us achieve that goal.
Fourth, if we ever relapse and fall back into evil, we should refrain from self-loathing, guilt and despair. We should acknowledge it as a setback and keep on striving—even with greater zeal—to implement these points and make a positive change in our lives.
Fifth, we should try to improve our knowledge of faith, and our understanding of sin and how to safeguard ourselves against it. Reading the answer to this question is a great start. Books of the Promised Messiah(as), such as Noah’s Ark, The Philosophy of the Teachings of Islam, and How to be Free from Sin, discuss the issue of sin at length. Similarly, books of Hazrat Mirza Bashiruddin Mahmood Ahmad(ra), such as Way of the Seekers and Ahmadiyyat or The True Islam, discuss these topics comprehensively.
References:
[i]. Sunan Ibn Majah, The Chapters on Blood Money: https://sunnah.com/urn/1269390
[ii]. Sunan Ibn Majah, Book of Zuhd: https://sunnah.com/urn/1293530