All these replies are very helpful!!
Estudiant9 - I do the exact same thing as YOU!! Some hot guy will be giving me a big smile and I just stare back with a blank look on my face and I feel like I'm frozen. I am very good looking to and I still have this problem.
How I feel anxious? I will notice a guy is interested and if I am attracted to him, I will start thinking "Oh my God, I can't believe he is interested." "I hope I don't do something to turn him off." that's pretty much what I think.
Secondary gain - I think I am too afraid of letting someone get closer to me because I think I am going to do or say something that will turn the person off and reject me. So I don't even give them a chance. I am protecting myself.