There are a lot of mental gymnastics involved in music.
Once you believe that you’ve made too many mistakes or that you are a horrible musician, you could start becoming horrible onstage. Once you believe that you’ve had no practice time or your playing has gotten worst, your playing could go downhill.
Sometimes, my older students start the piano lesson describing how little time they’ve had to practice or how poor their playing is. An element of that could be true: there’s no shortcut around putting in real practice time for improvement. But progress or decline from week to week shouldn’t be dramatic because our body has mechanisms to preserve motor and muscle memory. In fact, often the decline in performance is just happening in your mind.
That is what I tell myself as I demonstrate new pieces to my students.