Goddess--- I've seen a few different therapists in my day about this very issue. And one in particular gave me this piece of advice: Fake it til you make it. So, by pretending that you are calm and confident, you will make yourself moreso. Honestly, my problems with low self-esteem never went away entirely, but they did get better while I was trying to implement this philosophy. Just think of the most confident person you know, or think of how you know a confident person must feel, and then fake it for a while. After a few days, you'll start to naturally feel a bit more confident, which makes it easier to keep faking it, etc.
I'm actually one of those people that Lucky talked about. People meet me and seem to think that I've really got everything all together (seriously, just the other day a friend was telling me how she was totally intimidated by me because i have everything all together and I'm so confident)... It's so shocking to me that I exude that type of personality, because it really couldn't be further from the truth! But, it's a boost to my confidence when people say things like that, anyway!
One other thing that you might want to think about is where you're deriving your worth from. I have problems with this, too. For me, I feel like a good person because of the things I do. I'm good at school, and I do lots of nice things for people... those things make me feel like I'm a good person and like I'm worth something. BUT, the truth is, you're a good person because of who you are, not WHAT YOU DO. It's difficult to explain, I suppose.
When I was growing up, I felt like my parents loved me because of the things I did. I did well in school, I participated in lots of sports, I was basically a picture perfect child. (My parents were awesome, by the way... the best parents in the world, and I don't blame any of this on them...it's just sort of how I'm wired.) Somehow, I felt like I needed to do whatever I could to make my parents proud of me, or else maybe they wouldn't love me as much? Well, that idea really stuck with me without me even realizing it, and now, as an adult, I feel like my worth is still determined by my actions and my accomplishments. THE TRUTH IS, I'm valuable without doing anything at all. I'm a good person on the inside, even if i don't leave my own house! I'm worth decisions to eat better, I'm worth compliments, I'm worth listening to and looking at, etc. regardless of what nice things I've done today.
Long story, huh?
Basically, I'm just trying to illustrate that sometimes we get confused when it comes to our own worth. If you're deriving it from some external source, you might find that you won't really ever be enough... The best way I've found is to try to let your worth come from inside of you, that way it's not dependent upon any external factors, and therefore is more consistent.
Good luck to you! I know it's a really big struggle to fight your way back to getting that good image of yourself that you deserve. Think of it this way, maybe... it's your RIGHT to have a good self-image. Somewhere along the way, something/someone took it from you, but it's RIGHTFULLY yours. GET MAD! DEMAND IT BACK!!!