I'm copying the post I made to nessa, hope it will help!
The horse I ride is exactly like that. People stopped jumping him at my barn (he's a school horse) and basically only used him for dressage b/c he gets so fast. (He's a TB and both an ex-racehorse and an ex-foxhunter, so he has the speed thing implanted in his brain) I've spent a lot of time working with him, and we're now actually doing jumps w/out him landing fast and strong. Here're a couple of things we do:
-Don't give him a long line of sight. I'm assuming that you're jumping inside an arena - try setting up jumps on the short side or near the end of the arena so that they'll have to make a turn relatively soon after the jump. For some horses, seeing lots of space after the jump blows their mind.
-Don't give him a chance to anticipate. Switch it up; one time do the jump with a nice long approach, another cut across the ring so that the approach is different, another halt after the jump, another halt some strides before the jump and then trot it, switch directions every couple of jumps, etc. Try having several independent jumps set up so that you can do one than the a different one. If horses think that they know what they're about to do, then can get bored/independent and start getting fast. Often, if they can't anticipate they will listen to you much better
-also be very aware of the types of jumps that you do. Do things like bounces where they have to be aware of where their feet are, and so can't get super long and fast. Don't do lines with big distances (like 5,6,7 strides) in between the jumps, do in-and-outs and the such with slightly short distances so that the horse doesn't have the room to really stretch.
-you said that you tried slowing him with your posting; depending on both the rider and the horse, some people (like me) find it easier to slow them when doing a sitting trot up to the jump.
-Depends on the horse, but for some, like the guy I ride, cantering gets them worked up. We don't do an independent canter before jumping, just after/during jumps (I do plenty of canter work with him when not in jumping lessons and we give him a good warm-up at the trot, so this is ok)
-One thing my instructer has had me work on: instead of pulling on his mouth to slow him down before the jump, just hold steady pressure. Generally, we're taught to do half-halts and the like, but occosionally just holding steady pressure and not giving (obviously doing a small release over the jump and not keeping exceedingly hard steady pressure) works.
-be aware of your position, particularly your hands. Especially over small jumps, you don't have to do a huge release. Just make sure that you give a little, but you don't have to throw your hands at him. On the other extreme, make sure that you are giving them some release. Both giving them too large a release and pulling on their mouth over the jump often makes horses want to take off
-I don't know how honest your horse is, whether he's one that will refuse. The horse that I ride won't...so I can (as an excercise, obviously not all of the time) basically do a trot at an absolute snails pace and he'll go over, which helps
This is what I've been working on for a couple of months...sorry it's a lot, lol. Good luck!
