Some types of public speaking are easier for introverts than others. Giving a preplanned talk in front of a couple of thousand people? Pretend they aren't there and that you're speaking to the air. Look at the audience as you would look at a TV screen. It may not be the most spectacular or engaging talk they have ever seen, but you will get through it. Very few people are so interesting that you want to hear them talk for very long anyway.
A small interactive discussion group? That lasts all day? Now you are in territory where being an introvert is deadly. And there really are no good answers other than try to get some practice in small groups over short time spans and work your way up.
I think even that depends a lot of topic and format. I don't mind a technical discussion group at all. A social group on the other hand makes me want to just shut down. At parties I'll often just sit down somewhere and sit there all evening (unless I'm at a club - I like dancing and happily use that as an excuse for not talking), and don't mind talking if someone talks to me, but I won't seek out conversations unless it's a group of geeks talking about subjects I care about.
A small interactive discussion group? That lasts all day? Now you are in territory where being an introvert is deadly. And there really are no good answers other than try to get some practice in small groups over short time spans and work your way up.