I have no advice, But i'm with you. I have awful anxiety as well, especially at school, I often times throw up or feel nauseas or shaky. :(
I think you need to realize that you matter, what you have to say matters, and know when to say no. People don't respect us when we allow them to walk all over us, and often it upsets us because we really wanted to say no to them, and couldn't. I think the best way to start is by smiling and saying hi to people that you normally wouldn't. You may not always get a response back, but for all you know, you may have made that person's day! Many people are alone and feel invisable, your smile can carry them a long way and help you as well. The more you do this, the easier it becomes and often leads to conversation. If in line at the store make a comment to someone in line regarding anything from the crowds to the weather. Talk to the clerk, ask how their day is going. I have found that even the most unfriendliest looking people are eager to talk. This takes practice, and what's the worst thing that could happen? There is nobody more important than you, and you and what you have to say is just as important as the next person. Many people suffer from social anxiety and can hide it, and they are very envious of those who can just put themselves out there and not worry so much about what others think! Noy being social has not gotten you anywhere, why not try the opposite? I know it's easier said than done, but you can do this. I'm sure there have been times you've wanted to speak to someone but felt too insecure, next time just say it and see what happens. Be the first to laugh at yourself. Often if I am in a store and I bump into something or someone, I tell them I am like a toddler, and we laugh and go on. Think positive thoughts about yourself, we become what we think. I wish you all the best and take care. Write anytime.
Also note that I've been told that I don't take myself seriously and that I let people walk all over me.