I'm afraid it sounds like part of being a teenager.
I had a social anxiety problem for a really long time, but I basically forced myself out of it so I could get on with life. I was fine doing things like walking to the bus stop, and I was ok if I was with someone, but as soon as I'm alone, I started having problems. I still have this now, although I bite the bullet more and get on with it.
You have to look for the good in you, and concentrate on that. I used to always think of myself and tubby and geeky. Now I think I'm smart, and I've got womanly curves :P oh, and I've embraced my geekiness, although at uni I don't stand out so much for that!
I hated myself for years, but you really have to do the positive thinking thing. It does work, but don't expect instant results! It's taken me years, and I still do have trouble from time to time.
I'm sure you're a wonderful person, you just have to think that too.