What exactly did vet do to rule out pain?
Ultrasounds? MRI's? Cat scans? If none of those were used, than you simply cannot rule out this being pain related yet.
There could be all sorts of things going on internally that are bothering horse.
Would also imagine she is past age of being able to comfortably doing this as well. 15, while not ancient by any means, has with it bone changes, and body changes.
She may be anticipating the pain of having to compete, once you get to show grounds and just lose it too.
But I am with the others who say there is a reason horse was down for 3 years. And after watching her in the videos, I too am wondering if horse was actually competed on, as she isn't showing any of that in videos.
Find another job for her would be my thought.
Horses make me a better person.