Have you thought about chiropractic or acupuncture treatments? I swear by them.
In 1990 I bought the once in a lifetime showhorse (Secretariat's nephew). Less than a month after I bought him, he did yahoos outside in the pasture and really tweaked his back. He was unrideable. For three months I had vets working on him. I even accidentally blistered his back with one of the treatments. That poor horse was in so much pain. But worse, I just spent thousands on him and I couldn't even sell him to get my money back.
someone at the barn suggested a chiropractor, and I laughed. But she pointed to her horse who had a crosstie accident breaking her back. And she was back to 100%. So I called. The vet/chiro came out and performed a couple of adjustments for the first week or so, then we went to monthly adjustments. He hadn't been rideable in 3 months but he was back to jumping within 1 month after treatments. I did keep up regular adjustments for as long as I rode him, but I switched to every other month. That's not necessary but it helps keep their backs in alignment.