The key to overcoming social anxiety is creating that belief in yourself. This is actually easier than it sounds. Instead of constantly criticizing yourself and thinking bad of yourself, change the conscious and active thinking to good qualities that you posses and those that you could posses. And this is something that you can actively do. I am not talking about passive thoughts here. For example most of you probably keep saying in their mind "Oh I suck at that, I suck at this, I'm so bad etc.". Change that to the opposite. That's all there is to belief, that voice in your head that keeps negatively criticizing yourself is what creates that lack of belief. It is you actively doing it. It is your own fault. You cannot control emotions or what you passively think. You can however control what you actively think so change that to positive things. Believing is not some kind of mystical emotion. It is your own active voice in your head that says how good you are or can be. Once you stop being so hard on yourself you will realise it is ok to be afraid. It is all right to be embarrassed and many other things. Those are all normal things of life and all people have them, the difference is that you brood on them actively. Your belief will carry you into the unknown, and it will illuminate it. The more unknown you conquer, the less fear you will feel. But it won't matter to you because in the end once you start believing in yourself you will know that no matter how afraid you are or whatever happens in your life it will be okay because you know can handle it.
Anyway, if you have any questions on this or how to think in specific situations feel free to ask.