Does my boyfriend still have a thing for a past flame? Should I have ever gotten back together with him?

I have been dating this guy that I have known for 7 years for about 9 months now. For the most part I have been very happy with the relationship. He treats me very well and is very much a gentlemen. He shows that he really cares by always being there for me when I need him whether I need a study partner, a shoulder to cry or just someone to talk or go to events with. He never pressures me to do anything that I don't want to do, and he doesn't anger easily or put me down. From all that, he sounds like the perfect guy, right? Well, we had previously dated each other for only 3 short months before breaking up. We ended up breaking up because he quit calling and talking to me. I found out that during this time he was flirting with another girl on facebook telling her that he loved her and thanking her for dragging "two stray drunks to Perkins the other night." He also mentioned things like, "Where have you been all my life?" and "Will you just marry me and get it over with already?!?!?!" I don't know if he was just joking or if there was something more there because he doesn't joke around like that with everyone. He didn't know that I knew about all of this until we ended up getting back together almost a year later. I kind of casually mentioned her name in a few conversations before I told him about what I had seen on facebook, just to see what kind of reaction I would get from him, and all I got from him was that "she is kind of a weird girl," which led me to believe that he had been rejected by her at some point. After I told him about it, he first denied that he liked her, but later on told me that he did like her because she had big boobs and a really nice and sweet personality, but no longer liked her because he found out that she was a whore and was making out with another guy right in front of him when he took her to a party a few months earlier. He also told me that she had tried to get him to sleep with her at one point, but he refused...which I don't know if I believe. Immediately after he realized that I was really upset with the whole situation, he deleted his entire facebook profile and all of the wall posts from her. He also deleted her from his friends list before he reactivated and added me as a "friend" a few months later. Is he trying to hide something from me? On top of that, last night when we were driving in his truck we drove past the bar that this girl works at and I caught him checking her out in her car in the parking lot in his rearview mirror when we drove past! When I caught him doing it, he quickly stopped and his face looked a little guilty. I didn't say anything about it, but when we drove past again a few minutes later on the way home,he kept glancing over at the parking lot of the bar which he usually doesn't do. My heart just sank because I think that he still has a thing for her. He always tells me about how he sees me in his future and how he wants to marry me so should I be worried, or am I just too insecure? Is he only dating me because she rejected him? Does he still like her? Does he really love me as much as he tells me that he does? Should I have gotten back together with him in the first place? HELP! I will be moving 100 miles away for medical school in 3 months without him, and I am worried that he will find another girl and it will be over.