One summer at Blossom I overheard John Mack the former principal oboe of the Cleveland Orchestra say something to a chamber music ensemble he was coaching. "Don't worry about your great days, there will be plenty of those. Work on improving your bad days." I like that concept. You don't have to be having a great day to improve! That gives new hope and purpose for many discouraging spinning-your-wheels sessions. Yes, I may feel terrible, but that doesn't have to stop me from accomplishing something.
Progress often happens slowly. If you were to plot your progress over time, it likely would resemble the plains of Nebraska rather than the jagged mountains of Colorado. As we would make that long tedious road trip on vacations with the whole family from Ohio to Colorado, the little ones especially would become impatient with flat terrain. Where's the mountains, grandpa? Are we almost there, our 5-year-old grandson would ask? Yes, Andrew, we're making progress. But it doesn't feel like it, he complained!
True, it didn't seem like we were gaining any altitude at all, but it was happening imperceptibly as long as we kept moving westward. A simple thought, but maybe you can find some reason to take heart on many of those seemingly unproductive days. Improvement happens if we're steadily moving in the right direction. We don't have to notice it. We just have to move.
A couple of suggestions for such days: practice in short sessions, soft sessions, and sweet sessions. That is, play some of your favorite go-to music that cheers you, something fun. Soft playing is great therapy for the embouchure. Here's an ideal day to practice some of those pianissimo passages that usually get neglected on very active playing days. Don't get trapped into blowing a slug fest! Set a timer and stop playing. Rest the chops. Play little a lot, rather than a lot a little.
Find your own creative ways to improve when you don't necessarily feel like it. Take a weak area and break it down. Work systematically to bring that area of your playing up to a higher level. Rather than always practicing what you do well and avoiding your weak areas, decide to attack problem areas daily.
It may take a little time before you have a mountain top kind of day, so you may as well be steadily improving in the meantime. Enjoy the plains. Your mountains are just ahead! (Mountains indeed, but that is a topic for another day.)