This sounds a lot like the situation I was in last year, though I (full) leased instead of bought a QH gelding. I’m also a 30-something re-rider with about 1.5 years of re-riding experience. I leased the QH last April because he was supposedly a quiet, uncomplicated ride to W/T/C with. He quickly became terribly buddy sour, and it was a constant effort to keep him under control. As the workload increased (mainly starting to canter), he got dangerous. Refusing to go forward, bucking, rearing, etc. The behaviors were the same with my trainers who were doing regular pro rides.
I had the vet, saddle fitter, and body worker out for him. Late summer, he was diagnosed with ulcers, and I did about 6 weeks of treatment and time off. Re-scoped completely clean. Slowly brought him back into work and the behaviors were exactly the same for the following 4-6 weeks. Was there something else going on? Was it a learned pain response that was going to take time to go away? Was he just a jerk? I’ll never know because he wasn’t fun to ride anymore, and I was starting to be afraid, so I finally sent him back to his owner after 7 months.
In my particular situation, I didn’t send him back sooner because I didn’t want to be a quitter. I didn’t want him to end up in a worse situation. I didn’t want to be horseless. This seems silly now, but it took this experience for me to realize that it’s ok to not stick it out with every horse you ride. You’re not always a good match, and it’s too expensive (in money and time) to force yourself to continue doing something with a horse you don’t enjoy, or worse, are afraid of.
About a month later, another horse landed in my lap. She and I are a much better fit, and I am back to enjoying my time at the barn. I am so glad I finally decided to let him go so that this wonderful girl and I could meet.