Can you try ground exercises with your horses instead of riding? Teaching them (and you) how to do activities in an arena can strengthen the bond between person and horse. There are some free suggestions online and of course oodles of books/videos one can buy on how to do various training exercises.

If you want to ride, would a vest help protect you? Would you be able to find a small gaited horse that would be smoother to ride and a shorter fall down if one should happen? Could you find a horse that is as bomb proof as possible? Could you limit what you do with your horse, as in walking trail rides over smooth terrain or riding in an arena with soft footing?

I have the same issue. My horse is 16.1 and has a huge trot (former dressage horse). I rode a little after my diagnosis. Then ironically I fell in Walmart and hurt my back. I haven't ridden since then.