Why is her past hurting me so much?

You have to realize what is love. If you love who she is today then that is what you have. But what happend in her past made her who she is today but her past does not identify who she is, so you have to understand that and accept it too. When you say you deserve better than her, you are indicating that she is not good enough for you. If you feel that way, maybe you are confused with feeling sorry for her and be in love. I recommend you to ask yourself is what you feeling a true love or it's just your ego boost to feel better about yourself by being with someone you think lower than you.