Sexual partners

My boyfriend and I have been dating for about 7 months. We recently got to the sex talk. Basically we talked about how many people we've been sexual with in the past and the things we've done. I had met his ex girlfriend and now I can't stop thinking about him having sex with her. Sounds weird but I'm scared that if we have sex then I'll think of that and it'll kill the vibe. How can I help these thoughts leave? I know I have to accept the fact that I'm not his first because he won't be mine. Any suggestions?