Are you jumping just one fence or a course of fences?
We've had lots of horses like this but they settle down when they are given more to think about than a single jump with a gallop space beyond it.
Grid work helps a lot - starting with just walking over a line of poles on the floor and then when they can deal with that go to trot and then raise the poles to a low jump level always asking for a turn at the end rather than a straight run
We also lay out poles all over the arena and walk over them changing direction all the time sometimes just turning right around and back over the same pole. Do the same at trot once they feel they are calm about it at the walk.
You will never change a horse that is keen to jump (thats how you want them) but they can learn to be in control about it