In seven years I've never told anyone about my anxiety. Where do I start?

I can relate to what you are saying and i believe what you should do is open up and let the world see who you really are. I don't know why are you punishing yourself whether something happened or i don't know, but you gotta stop. You are wasting your life if you are not taking any opportunity you got at your hands. If teachers asks you a question raise your hand first, if friend invites you over say yes. If no one pays attention to you then make them, start talking to the people around you if you haven't done that yet, make them feel your presence. You have to be aware that everyone had gone through it only some people have done earlier than others. You have nothing to be afraid of, you are human being just as any of us are, if they can do it then you also do it. It's time for change don't sit there and wait for it, you are the change.