Try putting some ground rails on the ground before the jump. 1-3 strides out. Hope this helps
If your horses rushes, it means that he has a balance problem, you need to make a lot of work to put his weight back on his hindquarters (shoulder in, half pass, leg-yield, haunches-in...) and a lot of transitions so that he listens to you and is more reactive.
You need to make sure he doesn't have any physical problems (check with your vet) because if he rushes, it means he runs away from something and try to jump in an easy way without using his back.
You need to make sure also that you are not encouraging this behavior by beeing agitated in front of the fence, keep calm and wait for the jump.
Some tips that will help you are to stop him before and after the fence (you can even make him go backwards so that he will associate rushing with lack of comfort). Don't ever go if he rushes, make a stop or a volte. Don't stay hang on the reins, he will take support on it and rush even more.
You can work with ground poles at first, with distended reins to make him feel comfortable and safe over the jump, when he is calm on i, then try to jump small obstacles with a ground line for him to settle his stride himself.
When he's calm on that, work on mechanization lines, it will force him to stay calm (you can even approach the fence on the trot to begin).
I hope i could help you, good luck with it :)
I work with a horse that rushes and what I do is I talk to him in a calming voice I say things like slow down etc. I don't try to hold him, when he speeds up I hold him back for about two steps and give him a looser rain if he is still speeding I repete the "speed correction" when he reacts I, again in a calming voice good boy. It works because usually when the horse is speeding it is caused by anxiety, fear and or stress so do every thing to calm the horse by making it feel comfortable with you on it instead of giving it calming supplements. Talk to it and reward it for being calm (treats can work very well) generally make riding a pleasant experience.
One of the best ways is to put canter/trot poles before the jump to make the horse think and not rush
I’ve heard people say to work on stopping the horse before the jump. I’ve done that with my trainer before when her horses got speedy.