Should a young man marry a girl with whom he made a mistake in the past, despite his doubts about her conduct, or should he leave her, regretting what he did to her?

You must repent with a sincere repentance by abandoning the sin immediately, regretting it, and resolving never to return to it. You must also distance yourself from everything that leads to disobedience, and increase in seeking forgiveness and keeping good company. It is permissible to marry the woman, provided that both of you repent with a sincere repentance. If you are certain of the sincerity of her repentance, then do not pay attention to doubts, for whoever repents, Allah accepts his repentance. If she becomes your wife, you have the right of guardianship over her.