As someone who grew up doing Hunters, I jump the complete opposite of Foxhunter.
If you stay up in a two point the entire time it makes jumping really easy. Get up in your position, horse going forward from your leg. When you approach the jump the only thing you have to do is move your hands up the neck a bit and grab mane. You don't move your body at all. You don't go further into a jumping position, no need to catch up like you would if sitting.
Its learning to do nothing before you do something. Eventually you'll gain the control and skill to control your body and you can adjust your seat as needed. Maybe you do need a deeper seat with a certain horse or a certain fence, but as a beginner your horse should be uncomplicated and you can focus on you. You can see in this video the difference. The riders who take the fences in a half seat don't move.
Jumping without reins can help too. Its hard to throw yourself forward with nothing to balance on. You can do this one of three ways; being lunged over a fence, going through a grid/chute, or knotting your reins and letting go several strides before a single fence(harder).
I also find for myself, I let myself think about sitting more in the stride before the fence and that stops any excess motions. I don't actually sit, I'm still light and out of the tack, but it counteracts the compulsion to move the body forward. If you tend to duck, looking up and forward, like where the wall meets the ceiling at the end of the arena, can help.
But really, if you grab mane and don't move, from most positions you will then be pulled forward into the right position.