You've got to confront your fear.
I mean it depends what you mean by social phobia, but like let's say you are scared of getting infront of people and giving a speech. Do it on a small scale first, get through it, no matter how bad it is, book in to do another one. Then do it again and again.
Once you get it into your head that you have done something scary and come out of the other side ok, then it will make you ease up a little, and you will be less stressed and scared.
If you are scared of being around people in groups, try hanging out with 3 people first. Force yourself to get through it. Then arrange to hang out with 5 people, then 10, then 25, then 50. Soon you will be ok with going to big events and festivals no problem.