Definitely try a different bit! Something that has breaks when you want them but is inactive when you aren’t using it. I ride english but I know mylers are nice and I used an oval link elevator and that was really nice because it fit like a snaffle so it was comfy when my horse was behaving, but if I gave it a good pull the elevator action kicks in and put some pressure over his poll and in his mouth. More mouth than poll though but was very effective until we moved up levels and started going over bigger fences faster. he got more excited and we upgraded to a bigger elevator which could also be an option. Also, try having soft elbows but firm hands. This is effective with my horse because I can keep a soft connection so he isn’t uncomfortable but he has to accept his rein length and obey so he gets over it.
Bits can make a huge difference so I would switch it up until you find a compromise…one he likes but that works and has breaks for you.
The triple threat of riding = EVENTING! 😉