Why do I keep believing the lies of total screw ups?

You really have two questions, one about yourself and another about how to deal with people that may not treat you in the best way. In regards to yourself, why not try finding someone that shares your interests and with whom you could build trust as a friend. In regards to the girl you mention, it appears that she has a number of issues. Perhaps while treating her with respect, you could suggest that she talk to someone if she continues to treat you badly. Clearly, it sounds as if her issues may be causing her treatment of you.