You need to work on the flat, getting your horse to work from his hind end and not be on the forehand. You can use transitions, circles, half halts when needed and collection at different gates.
He also sounds like he’s not in front of and moving off your leg. So by collecting and lengthening at the different gaits, you’ll start to see how hnderpaced he is and how it will help you over fences. Do you have a coach? They should be a good source of info watching you from the ground. Good luck!
Whenever my horse does that my trainer will make me work on the flat until we get a good canter. A good canter is KEY to jumping. Usually horses are in the forehand because they do not have enough impulsion. I don’t know what you are doing to fix it at the moment, but I usually just kick on and give half halts. Another thing you can do it work on lengthening and shortening your horse’s stride on the flat.
Hope this helps :)