It sounds like you may have some sort of anxiety disorder. Like you said, it is a form of self-protection, the problem is that the sense of anxiety and urgency is in this instance for short-term self-protection from negative evaluation instead of your long-term happiness.

I personally experience something similar, but more it sounds my situation is more in reverse. I have a hard time approaching girls, even if they are interested in me, because I feel like they'll find something wrong with me or something they dislike about me, so starting up a relationship is difficult for me.

I don't really know what to say, as I haven't conquered my own anxiety problems. Although it hasn't personally helped me, Cognitive Behavioral Therapy is usually effective for anxiety disorders so maybe look into that as a way to break the cycle.