adam0408 wrote: After countless hours of critical practice and constantly telling yourself that what you are playing is not good enough, how do you switch to a positive performance mentality?
A negative mindset is, unfortunately, quite easily adopted when criticizing your own playing. Once adopted, it feeds on itself and becomes more ingrained.
Criticism is a good thing if it is offered and taken constructively. When working with a student privately, I first think, "What did they do well?" and point out that aspect of their playing first. Then I continue by saying, "But, here are some things we can work on to make it even better, yet." I then show them how to work on the aspect of the playing that will make them improve and help them develop a plan of action to improve that aspect of their playing.
I am sure there are things you like about your playing of the tuba or you would not be playing it in the first place. Think of those things and write them on a piece of paper. As you practice and you find more things that you are doing well, add them to your list. When you begin to have negative thoughts, pull your list out and refer to it. Think of your playing as that which is improving rather than that which is not good enough. Rather than beating up on yourself when you make mistakes, imagine yourself teaching a private lesson to someone else and think of what you would tell them to do to correct the mistake and improve. Detaching yourself from the situation will help you develop a systematic plan of attack to solve the problem in a more stress-free mindset.
Always remember that your proficiency level on the tuba is not a reflection of your worth as a person.