One of the many 'diagnoisis' I have recieved was 'social anxiety but as my therapist/psych got to know me I think they changed there mind. I was very socially anxious in my late teens. I wouldn't go out with friends, I was terrified to get a job due to bad past experience and I was scared of a lot of things outside. Like people smoking/doing drugs around me..so I wouldn't go outside. I didn't like using the phone etc etc.
I done two courses of CBT, the first course got me using the phone and helped me to deal more with going out then I 'relapsed' stopped doing the things that scared/challenged me and got scared again. The second course was basically a kick up my arse, reminding me I can do these things etc.
I wouldn't say now that I have social anxiety. Sometimes I feel awkward..but doesn't everyone?
Take small steps. Fighting the fear is really the only thing in my experience that worked. I just had to do it. After my first course of CBT I held down a job for a year and then decided to move to france for the summer and did that (and mostly it was ok) Then ended up unemployed (one of the reasons I slipped back) Then things started to go badly wrong again.
I am now back in employement, I do voluntary work with my sister (to help her with her Anxiety) so it IS possible to get past the difficult bit and hold down a job, a college course, whatever you want to do really.
I think the fact that you've started voluntary work is a great thing. Did you start this on your own accord or did you get help finding it? I think going to the GP would be a good idea. There are lots of charities and companies that help people with mental health problems/anxieties.
Mind for example run groups in certain areas. I also got referred although didn't end up using
'Richmond fellowship' Which offer a job club/support in work service.
So going to your GP is a good idea, even if you don't want a therapist because there are other options out there and he/she should hopefully be able to point you to them.
Good luck!