I can talk to anyone, and usually won't shut my damn mouth, but when I start to date a girl I just seem to close up. I know what my problem is, but I think you all can figure it out.
My suggestions is just start talking to people, absolute strangers, men or women. If you are at the grocery store in line, check out the tabloids then strike up a conversation. Also when you walk around make sure your head is up and you are looking forward, not down like a scolded dog. It will actually help build confidence in social settings.