I know a horse that is doing this because she is allergic to flies so maybe put an helmet and see if it's better.
I hope i could help you :)
I’m having the same trouble
Try a different bit. My horse had the same issue and we switched the bit and put a curb chain on. Also try to loosen your reins a bit. We found immidient results
I have this issue too, it's not related to pain though. If you find a solution, let me know 😊
Bit, saddle and spine
There are a lot of possiblities and causes. But if all the things that I listed abovedado are Ok, then it may be a related to behavior.