Regarding social anxiety: I was pretty much the same at your age, and I think the reason was simply a severe lack of self-esteem resulting in excessive self-consciousness. One thing that helped me was to realize that everyone is insecure to some degree, and that everyone is at least a bit worried about what other people think about them. Once I had realized that, I tried to make other people feel comfortable around me instead of worrying why I felt so uncomfortable. But this takes practice and time, which is why I still recommend seeing a professional. And when I say it takes time, then I mean that it takes time to to overcome the fear to do the things you're afraid of. Once you've done this a few times (or even just once), change can happen very quickly.
Try to worry less about people thinking of you as "the quiet guy". They probably don't think as much about you as you believe they do. They mostly think about how they appear to others, just more moderately than you (hopefully).
I'm confident that you'll overcome your anxiety sooner or later if you don't just ignore the issue, which you're not doing. So chin up!