I used to have exactly the same problem as you and i think it really just boils down to anxiety. I know it might be painful, but volunteer in school to do readings or present work. It’s not public speaking per se, but it does help when speaking in front of a group, however small or large that may be. Remember to breathe. I find that taking a few deep breaths before a speech makes me more calm and composed. Also don’t be scared to make mistakes. On the chance you do make a mistake, close your eyes for a second or two to recollect your thoughts, then carry on. When you feel confident enough, join clubs like debate or Model UN. You don’t need to be the best speaker to join such clubs. In my experience, they welcome everyone and senior members are likely to be willing to help others with their speaking skills.