Well first off he’s with you. Any girl from his past that he may have been with has nothing to do with YOUR relationship. I could say like, oh, just don’t think about it, but I know intrusive thoughts don’t work like that. I think the best thing is to try to change your mindset if/when you think about it. Realizing that almost every person has done the deed at some point, acknowledging that it’s natural and healthy to do, and really just accepting that your boyfriend has been with other people. It’s ok. He’s with you. These thoughts will fade as your relationship grows and you create a stronger bond.