i'm all against drugs, but you should try Xanax. It works wonders with some types of panic. But it shouldn't be a regular thing - just a support for you to enter the society.
If you started taking drugs and drinking at that age, you practically never developed a method to interact socially. So you're starting from scratch, and in addition to any anxiety that you may have felt at the time you started abusing substances, which was probably the reason why you did it, you now feel emberrased for being so inept at 20. And for gaining weight on top of it.
The fact is, you'll probably never going to feel happy in a large crowd of people you don't know; but on the bright side - the social ineptness is inevitable, but it resolves itself quickly if you just let yourself go.
You should just accept in advance that you're probably going to be humiliated, and humiliated big time; if you just accept humiliation in advance, you'll be able to overcome any of the issues you mentioned. And humiliation may seem like a bad thing only in your mind - in reality, the problems that you may bring upon yourself if you continue to shelter yourself are far more dreadful.
What the heck. If people like you - they like you; if they don't, they don't and there's nothing you can do about it.
