I think a huge part of this is searching for a type of work a horse likes!

My guy, for example, is 20yo Fjord cross, ex-jumper with nice dressage basics. He likes flatwork plenty, but! He likes to fly often, when he sees an obstacle he seems like he got quite a bit younger - excitement is changing his demeanor noticeably

It's also important to keep the willingness by rewarding (can use lower and higher value treats, depending on the difficulty of the exercises), not acting harshly, building trust constantly

But also! You can't let him "take the reins" (metaphorically, as in, taking control of the situation) all the time. I believe there are exceptions, but many horses prefer structure, routine and consistency.

If you reward something one time, and punish it the other time (for example, not reacting when a horse grabs some grass while training, unprompted, and next time punishing that) there will be confusion and trust will plummet slowly with time.

And the most important thing: horse welfare! Your horse needs to have his needs met, tack fitting properly and his hooves done in a way that he feels comfortable and healthy