I noticed that I was starting to dislike my own piano playing. I took a few days off, because hating your own playing doesn't make you play better. Even if the flaws you perceive are actually there, practicing out of that place does not help at all. I needed to stop and regroup for a few days.
Irrational causes of hating my own playing: I had accidentally set the dynamic control so that that I could not control dynamics any more. I played without noticing it, but finally it dawned on me. Once I restored the setting I like, it automatically sounded better.
Hearing people play better than me. Irrational because of course pro players will be better than me. I have no right to expect anything different. I should be inspired rather than intimidated.
Not thinking I have improved enough; not giving myself credit for improvement.
You can argue against the causes of depression, but not against the feeling itself. That is simply real.