I have been (and am going through) a similar situation. OTTB bought at 7 years old, jr eq horse for 3 years then switched to ammy jumper horse. At 13, started having creeping problems. First specialty shoeing. Then coffin joint injections, then chiro for a sore back. Ran out of money to diagnose so turned the horse out barefoot for a year. Pulled him out and SLOWLLLLY conditioned back (took around a year before jumping again).
Horse stayed sound for 1.0m+ jumpers from 15-17. Started seeing the same problems again. This time we went custom saddle, brought in a farrier from 2 states over to shoe him, watched the horse like a hawk. But over the course a year we started adding in hock and fetlock injections on top of the coffin injections and Legend before every show. It got to be too much and unfair to horse. So he is back out on Mr. Green at 18 years old. Maybe he will come back and play in dressage in a year, maybe not.
I am now half leasing a different, very low maintenance horse.
Long story short, don’t feel back about throwing in the towel financially. There is a problem with your horse. It could take big $$$ to figure it out or you could figure it out in the next $1k you spend on vet bills. I got to the point where getting on my horse gave me anxiety because I was afraid he’d go lame. I was constantly feeling guilty for continuing to push him.
I’m not saying my scenario is true for every horse but there is nothing wrong with trying some pasture board and see where that gets you.
You can only work with the knowledge you have today and the talent you have at your disposal i.e. farriers, vets. So don’t beat yourself up for not knowing or not trying. This is supposed to be fun!!