Bizarre question and suggestion. Has he ever been ponied? If possible, you might try to see what happens if you just drop the reins (this is why you would need a pony!). I’ve ridden a lot of racehorses (in training and retired) in the past, and they tended to go faster and faster when there was more pressure on the bit. Riding with a loose rein confused them and slowed them down. Eventually, they learned what I wanted them to do. Perhaps your issue is similar, dating from his past life? How is he if you just trot in open country and do not jump anything?
It is never the horse's fault