I have and sometimes still do, but in my case it is situational, not chemical. It is because I am living a new life somewhere else and it is scarey. This may not help, but it helps me.
Can you study yourself and try to get to some of the root causes of your anxiety? Would it be worry over a heavy work load, grades, or something else? Maybe write a note to yourself about it. What do you fear, and what do you think the worst that can happen is? Is it very horrible? What changed in your life to start the anxiety? What is different since it got worse?
Walter Cronkite advised a new reporter with stage fright to consider the worst that could happen to her if she fell apart on the air.. Would she die? No. Would her family and friends stop loving her? No. Would she be embarrassed. Yes. She went on the air thinking the worst wasn't so bad.
If you look at the anxiety more clinically and less personally, maybe it will help. Excercise and the breathing techniques Parker mentioned might help too. Good luck.