Why do I feel so ambivalent about romantic relationships?

I went through high school without having a single relationship. I had major crushes and a few close friendships that verged on relationships, but there was nothing official or physical about them. A few months ago I got my first girlfriend and was psyched for about three hours before I started to feel constrained and to worry. It didn't stop until I broke up with her a few weeks later. Now I'm lonely again and feel like I would like a relationship. Maybe she just wasn't the right person for me, but I feel like it's more complicated than that. I liked her more before we were "together".

For instance, I THINK that I would like to be in a relationship with someone I've really liked for a while now. Then I worry that I would lose interest as soon as we were together just because I'd freak out from the strong feelings, and I don't want to break up with him right away and hurt him.

Does anybody have a similar experience that could shed light on what I feel are my hopelessly tangled emotional issues?