<human>: I'm a good-looking 17 y.o. guy. I'm confident and good with people, so I have lots of friends of both sexes and quite a few girls have shown interest in me. When it comes to relationships, though, I can't get past some sort of wall in my head. I've hooked up with a few girls, but have never had a real relationship.

The past couple weeks, I've been talking to three girls, two of whom I'm almost certain want to go out with me. The third one is someone I've been friends with a long time, but we've been flirting lately. The problem is I can't bring myself to ask them out or really make any move that would take us beyond the realm of "friends" (however predicated on attraction that friendship is). Whenever I get close to being in a relationship, I tell myself I don't want to be in one, or I'm into someone else, or something else that I know is just an excuse, but is convincing enough all the same. If I had to guess why this is, I think I'm afraid to let someone know how I feel about them, even if I can see they are interested. 

Any suggestions on how to deal with this?

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<bot>: I find reasons not to take things beyond "friends" even when I'm sure a girl's into me. Thoughts on why I might and how to get past this would be nice.