You’ve gotten some great advice and I agree with what’s been said.
I have a young horse who was like this for his first few shows. In his case it was because he’s young and was super green at the beginning of the season and most definitely did not have all of the correct muscling built up to “hold” himself appropriately. He would canter nicely up to the fence and them leap over it with a ton of forward momentum and land super flat and pull me down to the next jump. I could always get him back in the corners, but the lines were pull, pull, pull, pull!
I did play around with bits and found that I really liked bits that worked on more than just the mouth (more specifically, I added a gel-cover topped curb chain to a simple D-ring Myler bit I use and found that the chin pressure/lift of the curb action helped him get the idea of lifting much better than just a mouthpiece).
But what really made the biggest difference was conditioning work. We started doing hill work and also played around with some exercises at home including:
>jumps with take-off and landing poles (sometimes one or the other, sometimes both) - 9’ before and/or after the jump. Also, we would often school a small vertical at shows with a landing pole 9’ after the jump (no take-off pole) as our last few jumps before walking into the ring.
>pole - 4 strides - jump - 4 strides - pole - this was probably the most effective exercise for this particular horse out of everything I did. It really seemed to resonate with his brain that he had to stay the same speed after the jump as he did before the jump.
>Bounces - I set up a line of 6 crossrail bounces (set at 9’6"-ish) and had all of my horses use that as a little warm-up exercise all summer. This was another that really helped this horse think about staying round over the jumps and maintaining a steady pace.
Doing those exercises (and focusing on conditioning) has made a world of difference for my guy who is now jumping around without getting flat and pulling me around.