Hmm. Alright, well bear with me.
I'm currently training a horse to jump. She's going to be 19 soon and we're almost at two feet in height after about 9 months of reconditioning and training.
I have also retrained multiple horses (the mare is included here), nd one of them was an abused showjumper. He got super excited whenever a jump was nearby. When at lessons, my mom (who rode him in lessons) would have to walk him around so he didn't know that it was his turn. I ended up riding him aftr she got too busy.
Anywho, what I found worked really well was trotting to a tiny crossrail (my foot wouldn't even fit under it) and then slowing to a walk, walking over the crossrail, and the halting about five strides on the backstretch. And constant reassurance.
I eventually got him to trot a small 12" crossrail and then halt almost immediately after it. He's now a lo more sane and my little sister (who has more confidence over jumps) is progressing him for herself.
But I would always treat a horse who hasn't jumped in a while as a greenie. Better safe than sorry.