When I got my mare, she was absolutely insane. She was just fast in general, but the second she saw a jump there was no hope of controlling her... What I've found to work is walking the jumps. I'll walk her up to a cross rail that's say, 18 inches. We don't usually go over it, but she has to walk. Then I turn her around and we approach it again at the walk. If she starts trotting, we stop, back up, and I wait until she's standing nicely. Then we walk forward. If she's being good, I'll let her trot the last stride or two and we pop over the jump. If she takes off on the landing, I usually let her go. But I'll push her as fast as I can get her to go because then she's still not getting what she wants. We redo that and if she takes off, I let her. I let her go 3 or 4 times until she's out of breath. Then the next time I make her stop after, and back up and stand. (No, that's not cruel. It's effective
) I don't know if that'll help you at all, but just my idea :)
Edit: I've also heard trainers say that when they have a horse that won't slow down, they raise the jump until the horse if forced to slow down. I've never tried it, cause I don't ride nearly well enough to do that, and it sounds a bit... I don't know, inncorrect? To me, but hey. If it works for someone else that's great.