How to Make Your Horse's Training Stick
I can't tell you how many times I've heard, "But my horse already knows how to do that..." People assume that because their horse has been trained to do something at one point long ago in its life, they will automatically have it encoded in their brain forever. This is simply not true.
Horses need repetition and reminder lessons often to keep what they know fresh in their mind. These lessons should also be done to let the horse know these things will be expected of them often.
No walk in the park handling a spoiled horse
I had a friend who had a very pushy horse. The horse was incapable of being ridden or doing intense groundwork due to arthritis from his racing days, so he rarely got worked with.
The only time he was handled was when he was being rotated from pasture to stall or being held for the farrier. Me, being the stable hand, was the one who had to handle him for these things, and let me tell you it was not easy! You try to handle a spoiled, 17 hand proud cut Thoroughbred who hasn't been worked with in 5 years. It was no walk in the park.
keep his manners sharp
I did what I could to keep him in check, but since he was not my horse I wasn't at liberty to give him the training he needed. I talked to his owner on multiple occasions suggesting things to do with him to keep his mind busy and keep his manners sharp.
I suggested she do some ground driving with him, some light lungeing or at least leading exercises and work on keeping him out of your space and responding properly to pressure. Her reply to this was always "Well he already knows how to do that."
The point I'm trying to make here, is that if a horse's training is not enforced it falls apart. That horse may have trained at one point to respect your space, but years of un-training took place where he was allowed repeatedly to barge into your space and go where he pleases.
Time matters in training a horse
In that sense he was retrained to think it is perfectly okay to invade your space and disregard your cues. Now the horse isn't a bad horse or behaving wrong, he's just behaving exactly how he was retrained to behave. It's nobody's fault but the owner.
To keep this from happening to your horse, brush up on all his skills on a regular basis and don't allow any "un-training" to take place. Whether your horse is ride-able or not, all horses need manners and a basic sense of groundwork.