Is it a bad ride? Or just not as good as the previous days?
Every day must be the same in our minds. It is so ingrained in human nature to just "pick up where we left off" the previous day, we must strive to eliminate this and start every ride from scratch.
Start with the training scale, rhythm. Once you have that then move to relaxation/suppleness, then contact etc...
Every day I get on my horse who is schooling PSG I start on 20m circles and easy lines, stretching forward in a working trot and canter. No matter what we make it to that day, whether we get stuck on single flying changes or trot serpentines, that is what I work to improve. If we make it to schooling tempis and pirouettes, then that is what I improve. Either way - we must use the training scale, work through the levels EVERY ride and then each day becomes its own. Each day we work to improve something - and hopefully end up with a better horse the next day, the next week and the next month.Training plateaus are to be expected as the horse builds strength and understanding, but there is never a bad ride, only different rides with different goals.
For me - I sometimes go to the barn after 14 hour days and if I am not feeling up to it, I start at the walk and work myself up to a ride, making it as far as I physically and mentally can handle. If my horse is feeling great I can do the PSG that day, if he is off we might not even go past the working gaits. But I know I still will have improved even one thing and tomorrow is a new day.