Does a young man's committing indecency with his current fiancée affect their future after marriage, considering their repeated remorse and repentance each time?

Deviations stem from abandoning the way of Allah. Islam has designated legal marriage as the sole path for relations between a man and a woman, prohibiting any other relationship outside this framework. The Prophet, peace and blessings be upon him, said: "I have not left behind me any trial more harmful to men than women."

What you have fallen into, committing an immoral act with this girl, is due to following the footsteps of Satan. Therefore, you must repent to Allah from it and immediately sever ties with the girl. Adultery is a grave sin. Allah the Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way."

The expiation for adultery is sincere repentance, through regret, seeking forgiveness, and resolving never to return to it. It is recommended to strive for lawful earnings, seek permissible work, and marry immediately if able. It is not permissible to associate with this girl unless she repents sincerely, for repentance obliterates what came before it. You must also increase in good deeds, for good deeds erase bad deeds.