So this catches my attention, and here is why. This is my belief and maybe I am all wrong. But I do believe this.
I have heard people tell stories of “what happened years ago” that a horse still carries with him. Except I don’t think horse’s brains are built that way. Yes, something that triggers an old experience in a particular situation can definitely bring out the past, if it happens fairly soon after the original incident.
But horses do not, by nature, dwell on the past. Horses live very much in the present, reacting to the present. In my experience, they may remember feeling upset, but I’m not sure they even have a clear memory of most bad experiences. Horses don’t do what humans do and build fables around what happened first, next, and then. They tend to react so suddenly, I’m not sure their brain has time to build a lot of environmental details into the memory. Just a vague idea that something upsetting happened.
And, new experiences overwrite the old ones in the horse brain. In short, horses, by nature, are easy to re-school IF it is done correctly, every single time a situation occurs.
On the other hand, if they keep repeating the same behavior, that starts to imprint into a habit that has no reference to a past incident. It’s just a habit - even the pinned ears and sour expression is a habit.
The more time that passes, the less original memory the horse has to reference, until it is no longer available. Horses are focused on the present rather than the past.
It is humans that provide a decorative explanation for a natural, if inconvenient, horse behavior that may continue out of repetition, rather than for a specific reason.
But horses can imprint new behavior all too quickly. Research tells us they are remarkably trainable. It’s like chronic stall-walking or cribbing - it may have started for a reason, but it became a habit that is no longer connected with the original reason.
The problem is not the original reason, but the habit. The real problem is that human beings seem to be pre-programmed to dwell on old memories and old stories. As animals are not capable of doing. It is the human that is attached to that old story that rationalizes why this keeps happening. The human narrative becomes the roadblock to change.
We can’t fix what happened in the past. But we can definitely address a bad habit.
I’d recommend forgetting that old story. Don’t keep repeating it. Do your best to erase it from memory. The old story doesn’t matter to your horse now. I assure you that your horse is NOT thinking of it, he doesn’t have a mind for it. The details slipped from his recall within an hour after it happened. Rather, he repeated his last behavior, and that wasn’t corrected well, so it drilled in the behavior. He is now repeating what he always does. Simply because that is what he has been doing for so long.
If this were my horse, I’d address the repetitive nature of the behavior. Change locations, change how he is handled, etc. & so on. Practice farrier behavior at non-farrier times when there is plenty of time to work with it, and reward gradual improvement. It may be tough just due to the repetition having gone on for so long, but horses are supremely trainable. Some creativity could go a long way to making things easier, even if the behavior isn’t fully cured.
I had a horse that was terrible about farriers starting from before I got him at age 8 … except it turned out it that he wasn’t, really. Moved to another state, and the next farrier practiced actual horsemanship. At the age of 10 when my horse started with the new farrier, very quickly he was standing patiently with the lead around his neck for every farrier session. Both farrier and horse informed me that my services as a horse-holder were no longer needed. They were doing excellent, and enjoying each other’s company on their own. 
). I try to be that rider who is sitting up there acting like everything is so easy and fine, no matter what idiocy is taking place under the saddle. I’m far from perfect on this, but keeping it in mind is helpful.
) I don’t think he’ll ever be a deadhead but he was silly again today with less warnings so something is definitely amiss… sigh.
