There was an interesting post on the Trombone Forum by a trombone player having a hard time getting over the frustration of not playing as well as he wants. There was a lot of good advice. My comment was:
It depends in part on the gap between your playing ability and the difficulty of what you wish to be playing. I think frustration is very counter-productive and therefore would look to conquer that well before the trombone music. I believe your mental state determines your playing. As goes your attitude so goes your performance.
How about collecting a few small wins? What do you play REALLY well? Play that for a while. Now tackle something harder. Maybe not so hard that you can’t master it after some good practicing in one day. It sounds like what you are frustrated over is playing something so far above your current abilities – something that cannot be mastered as quickly as you expect (would like). Okay, so it’ll take more time that you’d prefer. That’s life.
Be grateful about what you can play right now. Be objective about it and know where you wish to be relative to where you are now. Listen to McFerrin’s “Don’t Worry Be Happy” for 7 minutes.