Where to start....
Your penis is not an ornament for other people to look at, it is merely a part of your body like your face. Like faces, they come in all different shapes and sizes. I think any kind of sport would help you gain confidence in your own skin, this is more important than anything for making you feel comfortable about yourself.
Secondly, if you have any half way decent A levels you could apply to uni as a mature student, this would at least make you feel as if your life has some direction which I think you may lack at the moment. That would contribute to making you a more confident person.
The fact is that you are a 21 year old man who can't go around having hang ups about his penis size for the rest of his life. 2 and a bit inches is better than nothing. Just have fun with it.