I need some advice about my relationship?

I was with a girl for 15 months and it was pretty good Until about the 7th month I was with her and thought about this dare that she did. The dare that she did, which was before I was with her was lick nacho cheese off of my ex best friends stomach(we all worked at the movie theater and they had a cheese dispensing machine). I had a chance to get with her knowing that she did that and I didn't care because she was so beautiful. So I got with her and it was great. Then after seven months my insecurity kicked in because i fell in love with her and I started thinking about what she did with my ex best friend and it was killing me because i felt like i got seconds. So I broke up with her and then I really missed her so I got back with her. Recently I started thinking about it again for some reason and then i seen that she was texting a dude she use to have a thing for back in the days (she told me they liked each other) so I just broke up with her again. I would talk to her about it but all she ever does is make it seem like everything is my fault. Is it really my fault? What should I do?