I like to check out topics that concern difficult horse behaviors, because I am a trainer and can always learn some new techniques and ideas from all the coth members with varied backgrounds. Always a wealth of talent at your fingertips…
OP, if your horse is truly healthy and sound…does he go right when you lunge him? Will he go rt, if you line drive him in the long reins? I always try to break things down and go back to the simplest training exercises that the horse knows and can perform well to try to sort out difficulties.
You’d do well to read and take to heart all the ideas that the cothers offer to you.
first and foremost, know that horses have the most generous of natures and will do almost anything for us if they physically can…if they start doing something contrary you had better investigate what’s physically going on.