Fear or rejection of the unknown, of jumping into new social situations.

I suppose so

Nearsightedness in wanting everything to go perfectly the first time so much that you don't get practice at making things go well.

Other experiences support this hypothesis in my case

Fear of exposing myself to rejection, coupled with harder to describe feelings of low romantic or sexual worth.

yep

The feeling that you don't really know for absolutely sure that you want to spend a ton of time with the person you're flirting with, so you shouldn't follow through.

I don't want to get attached to someone that's gonna burn me! :(

Two things have helped me with this. The first is increasing my self-worth a little. You can probably think of men less physically attractive than you who have had perfectly happy relationships. Try to understand what makes them attractive people (I tend to think of this as "falling in love" in miniature). In fact, I've found this exercise of trying to see the lovable in other people is a pretty good one in general. Anyhow, you can do this on yourself too. You have plenty of good points, I guarantee it.

That's a very compelling case. Thank you. And, I feel more positive about other people now too :)

I have a mantra for it, even: "I would regret not doing it, therefore I will do it."

I guess it's time to pull up that backlist of people I have a vauge interest in... ;)