Retraining a ruined horse is HARD. It can take infinite patience, and the re-trainer has to learn not to take anything the horse does personally–as in he (the horse) is not trying to shame you or show you up.Actually, if he is pain as you ride him he IS showing a decent disposition–as in he is not trying to kill you OR running around so amok with pain that he is simply self-destructive.
Try and track down the pain. If he is not in 24/7 turnout try to find some with friendly horses, because sometimes constant turnout plus the ever invaluable “Dr. Green” can help a horse start to find his own way out of some of his pain.
If you decide to keep him you are faced with a CHALLENGE. This challenge will take a lot of patience and you will have to have to put whatever dreams you had of competing with this horse in anything in stasis. It may take years of patient work before he is physically strong enough in the correct muscles to even think of specialized training.
I have always trained my horses, either from too young to ride hard to dyed in the wool rebels against human brutality, according to “Schooling Your Horse” by Vladimir Littauer. This is the basic training all hunter, jumper and dressage horses need as a basis for more advanced training.
Since your horse is older and already has problems with being ridden, it is going to take you a LOT longer than his suggested timeline, which is for a 3 YO who has just started carrying a rider. It is Forward Seat training, and very non-abusive for the horse, no side-reins for lunging, loose rein riding for the first few months, very slow jumping training, all based on a logical framework which he got from a Chilean Cavalry Olympic competitor when Littauer was a spectator at the Olympics in Berlin in the 1930s(?), combined with Forward Seat and Forward Control (Littauer explains this in the book.)
For slightly more advanced but similar training I heartily recommend “Training Hunters, Jumpers, and Hacks” by Brig. Gen. Harry D. Chamberlin, who ran the US Cavalry School at Fort Riley. Chamberlin’s training program is longer in time than Littauer’s (2 years vs. 9 months) but Chamberlin goes further, to the borders with true dressage as it used to be ridden (not today’s competition dressage.)
If you follow either book, listen to your horse, keep an eagle eye out for any signs of physical discomfort from your horse, you have a decent chance of learning how to train a horse properly, a skill that will help you throughout your riding career. It will take longer than ideal with him, but I have uncovered hidden gems of horses under all the crud of improper training, improper riding, and the resulting deep seated physical discomfort of the horse. It just takes time, a lot of patience and non-abusive training and riding.
I did it this way. I started with a green-broke (as in 3 weeks) 5 YO just gelded Anglo-Arab, as a beginner with just 4 years of trail riding. I did not ruin that horse even though I was a rank beginner. Later, many years later, a local dressage rider/student leased him as a confidence builder, neither she, her barn manager, or her dressage teacher could get over how soft, responsive and willing that horse was. I, as a rank beginner, was able to train a very green horse without ending up dead or ruining my horse, ending up with quite a decent riding horse by following these principles.
Mostly without a riding teacher, I could not afford a trainer, and for many years I was completely on my own when I finally bought land.
Your path may be different from mine in technique and experience, but every true horseman/woman puts the horse first, before the rider’s plans and ambitions, tamping down their egos so they can truly listen to what the horse is saying by his actions, his body language, aiming always toward the goal of a horse that is truly willing to work with you when you catch him in the pasture. Listen to your horse, he will tell you when you are right and when you are wrong. Horses can be the best teachers IF the rider has a sound theoretical knowledge of the art of riding and LISTENS TO THE HORSE.
Have fun!