Was great at my job, now I suck

The first school year at my new school as a band director went amazing. I was getting compliments from everyone. I turned around the band program, tripled the band sign up numbers, and had students playing above their level. I loved my job and thought I was on my way to being one of the best at what I do. My principal told me she’s never had a band director as passionate as me. This school year, I can’t stand my job. I’d even go as far to say that I’m bad at my job. The kids don’t play as good. I was told by a colleague that the kids play so bad that I should cancel the concert. I miss super important deadlines. Even with a to-do list, I get nothing done. I dread each class, even though the teaching part really isn’t that bad once I’m in the flow of it. My principal is showing disappointment. This seems to be a pattern, possibly associated with my ADHD. I want to just settle into a job and grow old in it. Why does this feel impossible?