How can I stop being so Insecure?

I always tend to look at people as if they intimidate me or are judging me. WHY DO I ACT LIKE THAT. In my head they are looking at me like I am this person who is super weird. I would consider myself a shy sort of a personality. I don't open up to strangers. Today I went to a bookstore where one of my ex high school mates was working and she was pretty confident talking to me and all I did was look down and shy around like a moron and act all blushy blushy as if she was asking me out. And well I did'nt really talk to her much but when I came home I kept going on and on about her accent when honestly it was'nt that bad. She was infact doing pretty well with the introduction of the books and stuff. I wish I was more confident and stopped looking at the world as if someone or something is out to get me. How can I stop being so insecure?