All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.{C}
Not being certain that you will not return to the sin again does not affect the validity of repentance as long as you are determined not to return to it, as there is no correlation between the two matters.
Therefore, dear brother, be determined not to return to the sin and strive to keep away from the causes of falling into it again, and do not pay attention to the whispers of the devil that you will return to sin.
The devil whispers to the person that he will return to sin, and that he therefore, does not need to repent.
It was said to Al-Hasan Al-Basri: “We do not feel ashamed before our Lord. We seek forgiveness from sin then go back to it, then seek forgiveness then go back to it? He said: Satan would like that you feel this way; never give up seeking forgiveness.” [End of quote]
Shaykh Ibn Taymiyyah said: “This is why a person is commanded to repent whenever he sins. Someone said to his Shaykh: ‘I have sinned’. He [the Shaykh] said: ‘Repent.’ He said: ‘But I returned to (committing) the sin again.' The Shaykh said to him: ‘Repent.’ The man said: ‘Until when?’ The Shaykh said: ‘Until the devil is grieved’” [End of quote]
Moreover, Abu Hurayrah narrated that the Prophet said in a Qudsi Hadeeth:
“A slave committed a sin and then said, 'O Allah, forgive my sin.' Allah Almighty said, 'My slave committed a sin and he knows that he has a Lord Who forgives the sin and punishes for it.' Then he sinned again and said, 'O my Lord, forgive me,' and Allah Almighty said, 'My slave committed as sin and he knows that he has a Lord Who forgives the sin and punishes for it.' Then he sinned again (for the third time) and said, 'O my Lord, forgive me,' and Allah Almighty said, 'My slave committed a sin and he knows that he has a Lord Who forgives the sin and punishes for it; do as you wish, for I have forgiven you.” [Al-Bukhari and Muslim]
Nonetheless, the Muslim must not insist on sinning, because insistence contradicts repentance. He should take the initiative to repent whenever he commits a sin, even if he repeats the sin, and he should be sincere in his repentance to Allah.
There are five conditions of repentance:
1- To be sincere, which is to repent for the Sake of Allah.
2- To abandon the sin.
3- To feel remorse for having committed it.
4- To be determined not to do it again.
5- To repent before the person breathes his last.
Allah knows best.