My situation's similar to yours, where my main horse is my all around show horse.
Like posted above, you have to give your horse at least a day to just relax, so he doesn't start associating riding with boring hard work.
Also, during my rides, which last around an hour or less, depending on how he's working, warming up, and cooling down, is an easy time for him, so he gets that break. I probably actually only spend fifteen minutes or so doing hard core schooling on whatever I'm working on.
Don't feel strange, if your ride only last twenty minutes even. If you get on, and from the warm up, he's being awesome, don't work it past that. Short and sweet sessions, he'll learn quite a bit, if even more. Too long schooling one thing, when they've been doing it perfectly for the past hour, they just start shutting down.
If you're working something like lead changes, or spins, get three or four perfect ones, and let him be done. Pick up on it next ride, if he's right where he left off, then move on to the next thing, because he's got it.
If you don't feel like working something that day, then just don't do it. It doesn't matter to him at all. Keep it fun. If you're not in a mood to work something, then you're likely to not be as precise on it, which just isn't productive.
"You're just as sane as I am."~Luna Lovegood.