This will probably fall on “deaf ears” but I will offer my suggestion anyway.
I have a few of aging horses, and not so aging horses, that have a variety of problems. I have an ex-eventer that I acquired because he had mystery lameness that was undiagnosed si problem. The si problem was later identified and injected but since he’s not in persistent work I quit injecting him. Another is very lame most of the time because of poor shoeing in the past that left one of his feet somewhat twisted, has also foundered and recovered. He’s been on glucosamine/ chondroitin / devils claw/ yucca, for years and still walked around stiff-legged. Both of these horses almost instantly — within weeks — responded to daily doses of turmeric. The lame one actually can be seen cantering about the field sometimes when before he would only stiff-leggedly amble about. The one with the si problem hasn’t needed a chiro again.
Further reading revealed that turmeric is also an anti inflammatory as well as good for diarrhea. I have a mare that has been producing cow flops since she was a baby. She’s now 6. I tried everything so I decided to try turmeric on her, too. She also had a swollen hock that, although she wasn’t lame, had been there for months and hadn’t responded to Surpass or anything else. After 3 weeks on turmeric the swelling was gone. And her loose manure improved.
A boarder’s mare is on it, too, because her problem resembled the description of the OP’s mare. And it helped her too. Helped my horse that I was told has ‘kissing spine’ which the chiro said was a non-existent condition, he only needed bending and stretching exercises. Not something I have time to do and he has also responded miraculously to turmeric.
Currently 8 of my 10 horses are on it for one thing or another because the stuff is cheap and magic. YMMV
Here is a website with great info on it. http://www.turmericlife.com.au/