My boyfriend is still in love with his ex?

We've been dating for 6 months now and recently he mentioned one thing that really hurt me. It was about his ex for 7 years.. she broke it off because she gave him an ultimatum to marry her and he didn't. She married another man in less than a year, which my boyfriend told me he didn't expect to get so hurt finding out about her marriage. He told me during the first months of our rel. that he should've married her, but I was pive about it.. recently he mentioned it again, that if they were still together, they would probably be married by now. I was so hurt, I feel like I'm only a rebound, I just want to break it off, but I want to make sure I'm making the right decision.