I don't have a shred of it now but I can recall a time nearly four decades ago when I did as a teenager. The odd part is that I don't recall doing anything specific to get rid of it, that just happened as a part of growing up and getting more exposed to the world.
In retrospect I know why anyone has it. It's a lack of self confidence in a public setting and a fear of ridicule, sometimes called social anxiety. Teenagers can be very unfeeling to their peers and single out some to ridicule them viciously, which is why many teenagers have social anxiety.
Very often this persists into adult life, but mine didn't. I realized that I often knew more than many others and that my language too was better than most. I have spoken to a lot of complete strangers since then, held classes and training sessions (no, my main profession isn't teaching) and addressed all sizes of audiences. It's not my preferred activity as an introvert, but I can easily do it as well or better than most.
Confidence is the main ingredient. It forms the very basis of personality and can be developed.