First of all let me say that you are not alone with this condition.
I imagine there are tons of people out there who are scared to speak before a large group of people.
When I was in college, a classmate of mine was very scared of giving a seminar which was a part of our master's program. We used to kid him around and encourage him. It did no good. Eventually he went to a psychiatrist who prescribed something to calm him (dont know what it was). Anyway, he took that and finally gave the seminar.
I am into natural therapy somewhat. I believe herbs and other natural things can help with your situation. Doctor prescribed drugs can mask the symptoms, they will not change your condition.
I think there is a reason for your anxiety. I am not a doctor so I cannot say exactly what it is. However, I would like to suggest a combination of herbs and would like you to try them out.
Go to your natural foods store (like gnc or natures harvest) and ask them for two herbs: valerian root and hawthorn berries. Get them in liquid form and take a dropper full of each before you go to sleep. You can also take them before any social function.
The valerian is good for calming the nerves. The hawthorn is great for blood circulation and for the heart. A combination of these two should work very well for you. These herbs are not addictive, have no side effects and cost very little (about $20 for both)
if this does not work, then I suggest you see a naturopath who will not only understand your situation but suggest some herbs or homeopathic medicine to alleviate the symptoms.
Also, keep in mind that shyness is a virtue. I am sure you are very good at heart.
Best of luck!