You could try Yumove Equine, or you can discuss with the vet the possibility of joint injections. However, at 31, in a lot of pain, and is struggling to move her front knee, I would really start considering all the options and whether or not you can really achieve an acceptable quality of life. I'm really sorry, old age in an animal is tough, especially when their lives are so much shorter than ours.
My vet said there wasn’t much we could do for her, my mom wants me to use a more natural supplement than my vet suggested so I had to turn that down 😭