And I would add, try an equine sports massage therapist. They often give a free evaluation. An ESMT found something that the vet did not see or feel. A couple massages, and better saddle balance with wedges, problem solved - much happier horse.
I agree about the spurs or any type of tie down, tie up, or any other "thing" you could use to "make" a horse do anything. Absolutely the wrong way to think - the horse is just that, a horse, and only understands horse "language". Get a clinician to help you learn that.
As someone said, start from the ground, start at the beginning. that is where you will get longlasting, desired results. I know from experience how to do it the wrong way, and that is with spurs, a crop, or any other type of non-horse-language apparatus. I had to sell my previous horse and start over, the right way, and the results have been so much better (plus I've had a professional trainer help out when needed).
There is only one way to get what you really want - cooperation - and that only comes when you can interact in a way a horse truly understands. They don't use whips and spurs on each other!