I sympathize! I had a horse with thrush to varying degrees for almost 5 years no matter what I did. I was meticulous about her stall, food, and exercise and used thrush medicine as needed when really bad. She was trimmed regularly by a good farrier. Nothing seemed to fully get rid of it until she was diagnosed with an ulcer and treated with Omeprazole. The thrush cleared up as a nice little side benefit and no more problems with it. Depending on your vet, it can be really expensive to test for and treat ulcers. Mine used the approach of looking at very clear symptoms of an ulcer instead of testing and just trying generic ulcer med (omeprazole) for 30 days so the cost was affordable for me. Hopefully if you try this route you will have a vet with the same approach. Good luck.