How can I become more confident?

I'm no loser, I have a good attitude about myself for the most part. I know I'm pretty, I have led a very interesting life, so I have plenty of things to talk about. But I am HORRIBLE about showing who I really am to others. I have an awesome sense of humor around people that I know, and when I'm with them, I can express myself freely and with confidence even if other people are around. But if it's just me? I have the hardest, hardest time letting my true self shine through. Like right now, I'm going to a cosmetology school--FULL of girls, as if you didn't know. I've never gotten along with girls that well, but why should I let that stop me from doing something I want to do? It is SO hard for me to express myself around them... Firstly, just because girls are bitchy and they usually jump on the first stupid thing you do or say... And while I don't want to let petty **** like that bother me, it DOES. I am a nice person... Too nice, which makes me ultra-sensitive to the slightest mean comment. When people insult me, I lose all composure, I get nervous, and I can't think of anything to say in my defense. Instead, I take the insult like a b*tch and then feel like absolute **** for the rest of the day. Basically, I have trouble recovering. It affects the rest of my day, sometimes even affects me for days afterwards, where I too insecure to open up to people. I don't know what's wrong with me. I see other girls laughing and talking and having a good ol' blast in my cles (and they're all strangers to each other), and there I am, the quiet one that nobody pays attention to. I mean, if people actually gave me the chance and just came up to me and talked, I would be fine. The problem is, it seems like when people see that I'm shy, they just totally leave me alone and won't approach me at all. So then I think they think I'm weird or something, because they don't approach me, so I don't know whether to approach them or not... I'm afraid of being rejected if I put myself out there. And yes, I suppose the people that reject the "awesome" person I am just aren't worth my time... But I get rejected once, and it's like the entire world is coming to an end. I just can't recover. I get rejected by one person, and it's like a chain reaction. I start feeling nervous, I get even more shy and quiet, I become more afraid to express myself, and then other people that might actually be worth my time (if I were just acting normal instead of like a depressed freak) reject me too. I don't know what's wrong with me... I don't know what to do about it either. I've always thought myself to be confident, at least healthily so with a little dash of insecurity thrown in there... And I AM for the most part! I really am. But often it's like a roller coaster ride... One day I'm really confident, the next, CRASH. I don't know how to just be confidant all of the time. "Fake it until you make it," I've heard some people say, but I just don't know how to get myself out of the cycle I'm in. How can I build my confidence so I don't feel like such a big retard? I've always wanted to be a little more loud and uncaring about what others think... But I'm always afraid I'll say the wrong thing, and then be really embarred for it afterwards, and then everyone will think I'm the biggest idiot in the world. I wish I knew how to break out of my shell... Please help....