What should I do about my desire to marry a girl I love and who loves me, but who lost her virginity with someone else, and I am still affected by what she did?

Love between young men and women is a reprehensible act and a gateway to evil. Both parties must initiate repentance and sever the relationship. You are not obligated to marry her, but if she repents and improves her conduct, your marriage to her could bring great reward, for the Prophet, peace and blessings be upon him, said: "Nothing is seen for two who love one another like marriage." Strive to forget what transpired between her and the other young man. If marriage is possible, then proceed; otherwise, let each go their separate way.