Why do i push people away?

I'm a private person for the most part. I don't like sharing more information than I need to when I'm talking to people but for by no means am I anti social or really shy. I have no problem talking with people and saying "oh I love this," or "I'm so clumsy," or things like that. Its just when people ( most of the time its guys ) get too close or show a interest in me, I get flustered and I space out. Then I push them away. Why do I do this? I've had a reletively normal childhood ( no scarring abandonment issues or divorces ) actually a pretty good one but for some reason I still push people away. It didn't start until about middle school and its gotten better but its still there. So being this way makes it hard for people to get to know me, or for me to date, because they think I'm either shy or a stuck up ***** when they talk to me. I have friends and one "best friend" ( we were really close in elementary and middle school, but we go to different high schools and we don't live that close to each other :( ) but no one really close that I feel I can share everything with them...can I do anything to make myself less uncomfortable and be more open with people? Really frusterated and thanks to whoever took the time to read this :)