I had that same exact problem all through highschool, and it still hasn't gone completely away.
The first girl I tried to actually approach was a classmate that was really kind to me. I realize now that she had no interest in me at all, but her personality was just very loving and accepting to everyone. Near the end of the schoolyear, I gathered up my confidence and told her how I felt about her (well, tried to). I had a whole speech memorized basically of my thoughts about her in order, but when I actually got her to hear me out, I cracked and my words were jittery.. I couldn't even think straight. Near the end, I felt tears welling up because I knew it was too good to be true and probably looked like I was going to start crying at any second. I don't think that she would have even said yes if I would have said everything word for word.
The whole thing didn't affect me that much at first because she told me that she wasn't looking for a relationship right then and I thought, "well, maybe when she is, she will remember that I like her," but about a month later, I found out that she had starting dating this guy that had horrible grades, was loud/obnoxious, crude, etc. It made me feel worthless and I started to open my eyes and see how weak I was.
After awhile, I thought that might have just been a fluke, and tried two more times on two different girls, and got rejected by both, and the last one, even to this day makes me feel worthless.
I don't want to be like this forever though, so I am to the point where I figure, now or never.