What should I do concerning the girl I loved and with whom I committed acts leading to fornication, and who then repented, given that my mother refuses my marriage to her because she knows the details of the relationship? Should I marry her or leave her?

You must repent of the forbidden acts you committed with the girl. Repentance involves abandoning the sin, regretting it, and resolving not to return to it, while keeping your secret. You are not obligated to marry her, nor do you sin by leaving her. However, it is advisable to strive to convince your mother to marry her, as long as she has repented, explaining to her that repentance erases what came before it. If your mother insists on refusing despite her repentance, you may marry her without your mother's consent. But it is better to forgo marrying her to please your mother if doing so does not cause you harm.