It sounds like you’ve already put a lot of money, time, and energy into trying to help this horse.
In general, racing TBs are bred to run really fast while really young. Yes I know there are awesome TB breeders who prioritize long term soundness. But how is the average amateur buyer supposed to tell the difference? The numbers game means that there are a bunch of them in the middle who neither win races, nor stay sound into old age.
If you’ve ended up with one of those, IMO it is not your responsibility to pour endless amounts of money into them, nor are you required to retire them for what could be 20 years at the expense of your other goals. A horse who isn’t comfortable in his body isn’t going to be one you can pull out of the field to trail ride, much less let a guest ride. Decide what your limits are, and don’t let anyone lay a guilt trip on you if you decide to euthanize.