Calm down, relax. Do the same work as at home, think like nothing bad will happen :)
What does your coach or trainer say? That should be your first stop in diagnosing this. If you don’t have one, ask someone knowledgeable to watch you from the ground.
Maybe trailer him to some local shows w/o showing and practice some relaxation. Suit up, hang out all day, practice the warmup. Let him watch everyone else and you can focus on him w zero pressure.
What bit does your horse use, my horse is very strong as well and I use a Pelham when jumping but and eggbut when flating. I would also recommend poles and gymnastics to help slow him down and think about his feet.
Hope this helps!
Lunging when you get there or grazing may calm them down, also making sure to give them a beneficial schooling ride before you show, specifically the day before or early in the morning of - i also find half halts to be super helpful, halfhalt for one beat and release for another, constant pressure can result in them getting stronger and more agitated. Good luck!!
yeah i would first look at if he is different at home and in the competitions. if he is the same at hoe try at put poles before and after the jump so he has to think about where he puts his feet. at competitions you can start small and just after each jump take him down to trot so he can calmly go through the course
I would consider the reasons for the behavior. If you horse is different at home, maybe look into ulcers. If there is no health problem, try playing with different equipment and try to get him to respond to you