Try faking self-confidence. I walk around pretending that I don't care what people think, and I pretend that everyone loves me, and everyone cherishes me and it gives me the motivation I need to flirt or even just get lunch by myself. (yes, I am that big of a dork that I don't like eating by myself... issues, I tell ya... issues)
I used to be "little, quiet 'tangie'" as I just found out through a friend of a friend type deal. When this girl heard that I rode a motorcycle, she just simply couldn't believe that "little" (I'm 5'9"), "quiet" (I like to rev on ricers, especially when I'm on my bike) me rode a motorcycle.
Find a way to break your own shell, even if it means just acting. Hey, it adds a bit of variety, so that when people get to know you, there's all sorts of facets to work out.