I’m so sorry this has/is happening to you. I can’t imagine.
Even if you don’t ride, which really can take a toll on your body, even a short stroll when you aren’t conditioned for it, would you enjoy even just brushing a quiet, older horse that needs some attention? With owners that have both a retiree and other younger horses that ride, it’s often the retiree that gets overlooked in the form of baths, or a long grooming (more than the quick brush-over). I’m sure any owner would greatly appreciate having their older horse groomed and loved on for even a half an hour- and I bet the horse would enjoy it even more. If they’re older and quiet, you probably won’t have to worry as much about quick reaction times and getting banged up. It wouldn’t hurt to post something on this forum asking if anyone near you has an older horse that would like some attention (and treats!).
I hope you feel better soon and find a way to interact with horses again. Best wishes.