The only thing that I've ever seen produce consistent, quick results is publically performing as many times as you can, as close together as you can, until you become used to the fact that people are listening to you and paying attention. Until you've made all your mistakes in front of an audience and gotten back up there. The good news is the audience can be an audience of one and the performance can be in your living room - as long as you feel under pressure to perform well. That means playing for friends, family, neighbors, in the park, at the mall, at school, absolutely any place you can find someone to listen to you. And there it won't matter if you mess up in front of some guy you barely know or your band director or Aunt Carol. Keep performing as many times as you can and you'll get used to people hearing you, used to the pressure, and soon the cycle will turn, you'll start having some good public performances, and the next thing you know, as you start to play or sing you'll be remembering how you did well the last several time you performed. You can't practice your way to confidence, you can't think your way to confidence, you can only perform your way to confidence. Next time, the 2nd most important thing for your performance.
I've seen both Bob Dylan and Stevie Ray Vaughan performing as nervous as cats and sweating up a storm. Anyone can get nervous.