How do I make things work? (10 pts)?

My boyfriend and I are now on our 4th round of dating. We have been dating on and off again for almost 2 years. It always starts out really nice at first, but then by the end of the first week he seems to lose interest. His parents, and family love me. To me, I'm the ideal girlfriend. I got good grades in High School, I'm going off to college, I can take his sarcasm and jokes, I'd do almost anything for him. Him and I also work, He has one job and I have 2. The only thing I won't do with him is have sex because I am still a virgin, and I am not ready to have sex; plus I am trying to wait for marriage. He's been saying all the right things and keeps telling me he's falling for me hard, but how do I keep him from running off to someone else? He has a tendacy to go back to exs, but 3 of em he cant..and another one is psycho. So I'm the only one left and I really like him. Any answers will be helpful!

Thanks.

If you need more information, let me know.