I used to get flustered or at least mentally fatigued all the time when it came to writing/recording or performing music. Those sluggish days were more frequent than the dialed-in days, until my friend, who's a visual artist, went on a long monologue about the 'emptiness is form' mantra. It was actually very enlightening, and easily translatable to music. You need to spend a lot of time thinking about what it really means and why it's so important to art, especially music. Nothingness is what distinguishes the somethingness. Drums are so effective at giving a song form because there is a wealth of empty space between each beat. At first I would refer to that mantra if I was slumping, but I've realized life flows more smoothly in most musical settings if it's always approached with that mantra. Incorporating it into a jazz band setting not only makes the music really expressive, it makes it a breeze to play, even if you're having a bad day. There's obviously a limit to how much emptiness you can add, but for the most part you can fill less than half, or even more, of the bars that are yours and still sound good. It's hard to drop certain habits, like where the stops and starts are of your internal musical phrasing, but to play nothing takes no skill, you just got to force yourself to stop when you normally would have continued, or wait when you normally would have started, and then pick it up a few beats later. Treat the rests like the notes and vice versa. Flip the whole thing inside-out and make it like a photo negative. Not playing as many notes as you would have improves the odds of a good performance by the numbers alone. Committing to less is also useful if you get a brain cramp and the key or chords escape you momentarily. During those times I deconstruct everything to 3/5 of a pentatonic, take a breather, and let the song come back to me. That's another big part of a good performance: staying calm. Anything you can do to clear your head helps (playing music takes a lot of brain power and overthinking what's going on is overloading a processor that's already close to max capacity). Music is 50% mental and the rest is in your head. Briefly oversimplify the whole thing if needed. During a show, feeling cool and confident has so much to do with acting that way; playing it off even if you're having a rough go, making the audience and your band believe you're all good by your body language. It certainly does spiral out of hand quickly if you feed the self-critical/self-conscious thoughts.