Set up a single vertical (or crossrail) on the diagonal, and start by trotting over it. As soon as you land, halt strait after the jump. Don't let the horse wiggle left or right. Then, as soon as you feel that the horse understands what your asking, you can continue cantering through the corner. Hope this helps!
Trot poles before the jump make the jump small and gradually make the jump bigger
It's a good idea to do cross rails and trot poles in a trot, then a slow canter, then a normal canter. You could try a different bit, if you hang on to the reigns a softer bit would be good, you can also make him/her stop straight after the jump so he/she doesn't run off with you!
I hope this helps
Lots of half halts especially before the jump, also gridwork and cantering over poles helps :)
Lots of trotting to and if you can, from fences. Placing poles either side or the jump helps them stay in trot if they keep trying to canter. Grid work with bounces and striding poles are great too since it keeps them in a rhythm. I found with my boy who rushed like mad did so because he was a bit nervous, so slowly building the height of the grid and making him go slow boosted his confidence and built his strength to sit rather than go long. Cavaletti exercises are great too, Ingrid Klimke has great ones :)
my old coach has a horse just like this and what she would do was make him trot up to it and then halt him as soon as he landed on the other side