Will God answer the prayer of a repentant girl who supplicates to marry a young man she loved and who promised to marry her, but then they separated? Is it permissible for her to continue supplicating with the intention of marrying him in a permissible way?

Only Allah knows the unseen, and the response to supplication is among the unseen that only He knows. Allah grants the supplicant one of three things: He either hastens his supplication for him, or stores it for him in the Hereafter, or averts from him an equivalent amount of evil. One should supplicate frequently with good expectations of Allah and entrust all matters to Him, for what Allah chooses is best for the servant, even if he dislikes it. It is recommended to supplicate that Allah bless her with a righteous man who will assist her in her religious and worldly affairs.