Praise be to Allah.

A Muslim may sincerely repent from a sin and yet, because of weakness, temptation, or the whispers of Shayṭān, fall into that sin again. This does not mean that repentance is useless, nor does it necessarily mean that the previous repentance was insincere.

The important matter is that whenever a person falls, they should not despair of the mercy of Allah. Rather, they should return to Him again with genuine repentance.

Allah تعالى says:

“Say, ‘O My servants who have transgressed against themselves, do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the All-Forgiving, the Most Merciful.’”
[Sūrah az-Zumar 39:53]

True repentance includes feeling regret for the sin, stopping the sin, sincerely resolving not to return to it, and—where the rights of another person are involved—restoring those rights.

However, a person may sincerely intend never to return to a sin and later become weak and fall into it again. If this happens, they must repent again. The door of repentance remains open for as long as a person is alive and the conditions of repentance are fulfilled.

The Prophet ﷺ narrated that a servant committed a sin and said:

“My Lord, I have committed a sin, so forgive me.”

Allah forgave him. The servant later fell into sin again and again returned to Allah seeking forgiveness. In the end, Allah تعالى said that His servant knew that he had a Lord Who forgives sins and takes people to account, so He forgave him. [Ṣaḥīḥ al-Bukhārī and Ṣaḥīḥ Muslim]

This does not give a person permission to deliberately continue committing sins while assuming that they can simply repent afterwards. Rather, it teaches us that no matter how many times a person falls, they should never stop returning to Allah.

At the same time, sincere repentance should be accompanied by practical effort. A person should try to identify what repeatedly leads them to the sin. Is it a certain friendship? A place? A website or social media account? Excessive free time? Being alone? Weakness in worship?

Removing the pathways to sin is part of striving sincerely against oneself.

The Prophet ﷺ said:

“The intelligent person is the one who holds himself accountable and works for what is after death, while the incapable person is the one who follows his desires and merely wishes for things from Allah.”
[Jāmiʿ at-Tirmidhī]

Therefore, if you fall into the same sin repeatedly, do not allow Shayṭān to convince you that there is no point in repenting. Repent again. Seek Allah's forgiveness again. Strengthen the causes that bring you closer to Him, and weaken the causes that repeatedly lead you towards the sin.

A believer is not defined by never falling. Rather, one of the distinguishing qualities of the believer is that whenever he falls, he knows where to return.