Can you explain more about what you do to keep going and when you decide to quit? What is he doing when you finally quit? What does he do between initial tantrum and quitting?
Some horses are more fighters. There’s a real art to finding the right time to stop for the day. My horse throws tantrums sometimes. They often come with a warning, but not always. They were the most frequent when he was rehabbing. We’d get towards the end of our exercise set, he’d be getting tired, things were getting harder, and he’d toss his head and leap and squeal and whatever. He wanted me to pull him up. Oh good, we get to stop now? It’s hard to handle this in a rehab where you are maybe at the trot for 10 minutes stage. But I’d kick him on to go through it, go back to trot…but maybe for only 2 more laps of the indoor. Then quit. If I was supposed to go for 2 more minutes? Nope, not happening today. Get a pleasant 2 laps of keeping his stuff together, then ok, fine. Push it more, and I was asking for a huge fight. Horses do not learn from the addition of pressure but from the release of pressure. So, getting tough and saying no, you WILL TROT 2 MORE MINUTES was not going to really teach him anything. Instead, say, no, you can’t do those moves to get to stop, but if you are nice and responsive and calm here for a second then, ok, I hear you, you can rest.
Usually, when he’s about to throw a tantrum because things are getting hard and he’s not sure he wants to do it anymore, post-rehab, I will get some warning. He will get kind of balled up in his body and start whining to me. Literally. This little whiny whinny grumble thing that he only does when he’s mad and wants to take his toys and go home. There’s a very short window between where he’s still trying really hard and learning and the start of the tantrum. I try to take advantage of that window then ask for him to relax and stretch. But it’s also my cue to wrap it up…don’t get greedy! Timer has run out! There’s really only so much good telling him to suck it up will do for us.
In other words, there’s a point in which just “making him work more” makes him say, “oh yeah? bring it!” But at the same time you don’t want to give into the bad behavior. It’s really an art to training ones like this. Variety in the type of work you do, even in the arena, can help keep him interested and less likely to hit your buttons. The real art is convincing the horse that he wants to do this sport stuff too. That’s not always easy!
I’ve also found that working the horse on the ground and intentionally getting him too up, then asking him to bring himself back down, rinse and repeat, gives him better control over himself. But some of them like to test you, especially if they are bored or would just rather be doing something else. I think there really is something to his added turnout that has made him think working in an arena is not as much fun as when he was more cooped up. But that’s what you’ve both got to learn to deal with.