Well if the disability is primarily mental with some physical expression, and you are already in a riding therapy program, and you are actively transitioning back to the workforce, I can’t see riding being an issue with your benefit provider.
That’s a bit different than if someone was claiming a purely physical issue like a bad back kept them from ever working again, but they were spending all day doing barn work.
As far as owning your own horse again, as I’m sure you know horse ownership can entail a wide range of cost and physical effort, from pasture board to self board to full training board which is very expensive but good if you lack time or energy.
On the other hand paradressage horses are very special animals and my guess is they are expensive and require the full time help of a trainer to keep tuned up. If you want to compete it might be more economical and achievable to lease a horse in your trainer’s program.
Indeed most able bodied ammies who want to compete also do much better in a training program!