I’m very, very sad and mad that your girlfriend has been so badly mistreated. But at this point, your problem isn’t with the various men who have hurt her or had sex with her. Your problem is that your girlfriend keeps putting herself in situations where she will be victimized and then possibly be rejected by you, the one guy who truly cares for her. This isn’t uncommon: Sometimes when women have been sexually abused, they feel so bad about themselves, they think that somehow they deserved it. Their self-esteem is so low, they don’t stand up for themselves or leave situations where they can be hurt again. Other victims develop the idea that there are only two ways to be in a sexual relationship: Either the victim or the victimizer. They never want to be an abuser, so they repeatedly get into relationships where they will be the victim. As hard as it may be to understand, all of these types of problems are mistaken attempts to process the original abuse or rape.
You can’t do anything about what guys did to her but you can help your girlfriend. She needs to see a therapist who specializes in trauma work. Encourage her to make an appointment and get started in therapy. Until she gets the help she needs, she will probably continue to do self-destructive things to test your love — like sleeping with your former best friend. No one can blame you if you can’t stay in such a relationship. But if she does do her therapeutic work, she will be able to put the sad history where it belongs – in the past — and she’ll be able to see all the worthwhile things about her that you see.
I wish you both well.