@SLS I don’t know how much you have investigated this diagnosis. But you sound so heartbroken, and sad that I thought I’d respond, since not a lot of others have done so.
High ringbone has a better prognosis for future riding soundness than low ringbone, so that is hopeful for you. It really depends on the extent and location of the bone growth so far, and WHY it happened in the first place. Your vet can tell you more about the first part, and maybe some idea about the second. Sometimes knowing this can help restore some level of soundness and usefulness for the horse. “Why” can be overuse, concussion, farrier skills, injury from a blow, conformation or something else that I’m not thinking of at the moment. But try to figure it out so that you can avoid having this happen again.
Arthritic changes in a joint may do any of a number of different effects on the horse… When fresh, they will cause pain. When they set and cool out, they may not be as painful. It is possible that some bone growth will wear smoother in time. It is possible that the joint may fuse, or partially fuse. Full fusion can happen naturally, in time, or can be encouraged with veterinary intervention. Partial fusion may mean only part of the normal flexion in the joint, but may be OK for light riding. I heard that one horse locally was doing the 3’ hunter division successfully with a fully fused hind pastern.
“Supplements” don’t cure arthritis or make it go away. Painkillers and anti inflammatory drugs may be helpful, may be called for, and do work as advertised. Your vet should have some insight into what may help your horse return to adequate soundness for light, recreational riding. If your vet doesn’t have this information for you, get a second opinion. Owning and riding and training horses is always a series of set backs, injuries, ailments and disappointments, it rarely goes smoothly and the way we all want it to go. Welcome to the club. Horses who have had high ringbone diagnosis sometimes do come sound again, given time, one way or another. I’ve had one who did come sound, and resume his show hunter career, and it was in the days long before the joint injections that are available today. So… keep your chin up. Good luck.