Why am I so scared of being happy?

A little background: I'm smart and am told I am pretty. I have no problems attracting guys or making the right grades in school. I got out of my third bad, consecutive "relationship" in June. All three "bad endings" involved another girl behind my back.

I am currently dating a wonderful guy. He's sweet, smart, funny and considerate. He cares about me. All this being said, I start to envision how things will go wrong. I assume they will. In fact, I'm already envisioning how he will tell me he cheated and how I will respond. If it's not that, I think of ways to "detach" myself from him. I've thought about breaking things off on more than one occasion because I think I'm starting to care too much about him. If I can leave first, I can't get hurt. It troubles me that I have these thoughts. I just recently met his two best friends and found myself really attracted to one of them. WHY do I have these terrible thoughts? I like him a lot, but I can't deal with more than I can bear to lose at this point. I thinking I'm screwed up. any words of advice?
Why am I so scared of being happy?
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