Jamey Aebersold gave a great demonstration, which was so revealing to me as a jazz teacher.
He assembled the students in the auditorium at University of Louisville. He would pluck one “lucky contestant” from the crowd to join him on stage. Jamey would then hand the musician a microphone and instruct them to sing/scat/hauler a jazz solo along with the chords he would randomly play on the piano.
Everyone could scat, some very well, others so so. But the consensus was that the “singers” could scat better than they could play. Hmmm. So, the problem isn’t in the head, it’s in the hands.
Conclusion 1: Jamey was encouraging everyone to improve their general musicianship skills on and off the instrument.
Conclusion 2: If you are a pianist or guitarist, sing and play at the same time.
Jamey's second reveal.
One morning he asks the musicians, “How many of you can play 50 jazz standards from memory?” I enthusiastically raise my hand. Looking around the room of 250+ there were very few hands joining me.
How many can play 25?
Overwhelming the answer was zero.
Conclusion #3 It’s hard to play freely when your head is in a book.
Conclusion #4 These musicians didn’t trust themselves to play without a book/sheet/app in front of them. He was happy to sell them another book.
I hope this helps you begin to think through what may be holding you back.
You've got to learn your instrument. Then, you practice, practice, practice. And then, when you finally get up there on the bandstand, forget all that and just wail.
I'm a professional pianist and music educator in West Toronto Ontario. I'm also a devoted percussionist and drum teacher.