Jesus mate. I don't think you need to go see a doctor. But your current course of fixing the problem is obviously not working.
Try reading aloud. Not going to be a cure all, but it will help you get used to speaking in class.
I'm pretty introverted and while I knew a lot of the answers in class and had the ability to offer discussion, I wouldn't. Not out of fear, I would just come up with the answer in my head and move on kind of following the discussion mentally. Reading aloud got me used to hearing my voice so that actual class participation became normal. It really helped me out in debates and discussions (many of my profs took that sort of thing into consideration for final grades).