Hey, I can relate a lot. I really struggle with social anxiety as well, and have done for most of my life. It isn't something that I've grown out of and it's difficult when people seem to expect that you should. But it is something I'm learning to deal with and although I still come across as really shy a lot of the time, I can do things now that would have seemed impossible not that long ago. I don't spend so much time beating myself up for saying the 'wrong thing' and I'm more able to talk to people without freaking out. So there is hope!
CBT was really helpful for me, and I know a lot of other people also find it very useful so I think asking your GP for a referral is a really good idea. Also, there are things that you can try independently; I wouldn't say they should be used
instead of actual therapy but might be useful alongside it or while you're waiting for a referral to come through. I found the book 'Overcoming Social Anxiety and Shyness' by Gillian Butler fairly helpful, and there are online CBT programmes you can try.
ecouch was recommended to me for social anxiety. I never really got into it so I can't say how good it is but it might be worth a look.
I've found it useful to keep pushing myself to do things that I'm scared of. It can be really difficult and I have had to accept the fact that sometimes - okay, a lot of the time - it's going to make me feel rubbish at first but that it is worth it. Starting with the smaller things and working up can be better than throwing yourself in the deep end. It sounds like you're already doing that, with starting volunteering and such. That's really great, and I hope you're able to keep doing it. Instead of focusing on how hard it is to enjoy it because of the anxiety, if you can try to remind yourself how great it is that you're even doing it. That really is an achievement and you should be proud of yourself. And chances are, if you keep doing it, it will get easier and less anxiety-provoking and then maybe you'll feel ready to try something else challenging. I have found volunteering really helpful; there have been times I've walked home in tears because I felt completely useless and hardly said anything. But it's easier now and I'm glad I've pushed myself to keep doing it.
I'm really sorry for rambling! Really just wanted to say that I understand some of what you're going through, and it's awful, so I really hope things get easier for you. Take care of yourself.