you could work on transitions as you flat, which helps with halting after jumps and making the horse pick up the trot when lading after the jump
-Make sure before the jump you aren't squeezing too much, and not squeezing while in the air.
-In the air, over the jump(and before) also tell your horse "woah" to tell them it's okay, you can stay slow.
-and when you land say woah again. Make sure you SIT UP straight, and pick your hands up about 2 inches.
-when you land make sure you aren't pushing for a canter, so straight into a post.
-and when you post make sure it's NOT a fast post. Do a very very very slow post.
Do this multiple times for multiple rides and see if you start to get a difference
You could try putting poles down after the jump that way your pony should be more focused on the poles rather than the speed. Hope it helps!