Fear of public speaking often stems from the speaker thinking that the audience's main objective is to criticize her performance. The opposite is true: the audience wants you to succeed. First of all because of selfish motivations; nobody wants to be bored.
But there is an emotional driver as well. People (in the audience) do not like to subject themselves to an embarrassing situation. Watching this movie clip from the film "About a boy" creates that exact feeling in your stomach (I cannot embed it for some reason).Confessions of a public speaker" has a great section on public speaking anxiety. Seth Godin thinks that fear of public speaking is the a prime example of our lizard brain at work.