Is a young man considered sinful if he repents from a forbidden relationship (whether by phone or in person) and promises a girl marriage, but is unable to marry her for many reasons, and the girl has refused to forgive him?

You must take the initiative to sever ties with that girl and repent to Allah Almighty by abandoning the sin, regretting it, and resolving never to return to it. Do not pay attention to what the girl said, for she has no rights over you as a result of the forbidden relationship, and you are not obligated to marry her. A suitor is permitted to withdraw from an engagement for a valid reason, so how much more so if the promise was made in a forbidden relationship? Therefore, turn away from her and be sincere in your repentance.