I experienced something similar but with arthritis. I couldn't bring myself to ride for a year or two after. It helped me to know that guilt is part of the normal mourning process and eventually you'll be able to move on to the next stage and things will start to get easier. Go easy on yourself...
Recently I have been riding green horses in my riding lessons and my instructor recommends doing a half-seat type thing (lifting your weight out of the saddle a bit) which allows them to use their back more easily. You can then also use your seat to send the horse forward before returning to...