As far as self-esteem goes, "fake it 'til you make it" is not a bad cliche to adopt. Go to some club or bar off-campus where you don't know anyone and where you can feel relatively safe in experimenting, and just "act" like you have self-confidence.
Before you go, in your mind's eye, imagine an actor who looks and sounds exactly like you, except that he interacts with people in the confident, self-assured manner that you aspire to. Play around with that image in your mind until it's clear, and then go out one night, and "play" that character in real life, as if you're doing an acting exercise. It will help you start to develop a new way of interacting.
Be patient with yourself, though, if you're used to carrying yourself a particular way, it won't change permanently overnight, but you can train yourself to develop a different physical presence, and that can help shift your mental attitude.