He told me about a girl in his past that it didnt work out with, would consider taking her back?

My boyfriend and I have been together now for about 6 months, a while ago he during a post-bar heart to heart, we breeched the 'ex' talk, and he told me there was a girl right before me that he had liked a lot but that it didnt work out with. She did not like him back. He was very taken with her, and I'm pretty sure was in love with her off the bat. He said he ended things with her when he realized she wasnt reciprocating his feelings. I asked him if she changed her mind now what would his response be (considering were together now) and he said, if she begged for him back he might consider it. This obviously didnt sit well with me and I explained to him that I didnt want to be anyone's second choice or emotional crutch. I told him that would make me rethink things. He got very upset and cried and told me he was happy with me and he felt like he messed things up by saying what he said. The next day, he said he spent the day thinking about us and that he needed me to know that he did not mean what he said-that if things were to end with us, it would not be because of her coming back into his life. I recently found out that back when we were a month into dating he had gone to visit this girl and try to sleep with her. It didnt end up happening because she didnt want it to, and shortly before him and I became 'official' he tried to reach out to her and talk with her because things were getting more serious between him and I. After she didnt respond to him, he sent her a message saying that he was giving up and wished her luck. He hasnt spoken to her since. Although I trust what he says and am happy with him, I cant help but think about the fact that its possible he may have deep feelings for someone else and that his relationship with me is helping him forget that, especially since she is the one that essentially ended things with him, he just 'got the hint'. My fear is that she seemed to jerk him around a lot whenever she was lonley and he always came running to her. What are the chances this could happen again in light of our relationship?