You sound exactly like me! I'm really shy and quiet, but once I have actually properly got myself into conversation with people I'm comfortable with (or anyone if it's a subject I'm passionate about - weirdly, I have no shyness issues at all if it's a topic I feel strongly about, I will start arguing with anyone at all!). I also prefer to keep to myself in public rather than have to stress myself with social situations, even though I'd rather be with people, it's too much hassle. And I always feel pleased with myself if I've put my hand up in class and volunteered an answer or opinion, or talked to someone I don't usually talk to, it's weird how that's such a confidence boost! Just try to do that as much as possible, the little things like that, to try to build up your self-esteem in social situations.