I was always taught never to let the horse leave the jump. If it ran up to thr jump and then slammed on the brakes I was to make it go over the jump no matter what...even if it crawled over and knocked down the poles as it did so.
Think of it from the horse's perspective: it knows when it stops in front of the jump you turn it away. That is the instant release and reward this horse is looking for. He effectively just got out of work.
My advice to you is to work on smaller jumps or crossrails you can put the horse over no matter what. You as the rider should be sitting tall, looking on past the jump and asking for impulsion while keeping the horse straight and well controlled between your hands and legs.
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