Try really sitting back after the jump and half halt every time your horse takes another stride until your horse slows down
Could be that you have too much leg over the fence, or if you have a long distance that naturally produces more energy on the backside of the jump. Jumps alone produce energy after so that could be what you are feeling. Halting after can help if you feel the horse runs after the jump, or try seeing what happens if you take your leg off over the fence.
Put poles down before and after, work on grids/gymnastics, and only walk/trot into the jump and walk/trot/half after
Halt after each fence or circle. Also in this case flat work is key.