It's going to take a little while to relax and follow the movement of the horse. Having a flat gaiter lesson horse (like many TB's, etc) can help you learn to sit. Otherwise you're going to have to find a way to relax if you are riding a horse with a lot of impulsion. It took me a long while to learn this and to be honest, since every body (literally your body) is different, there's no quick way to do this. You have to feel it, get a feel for the moment--ride different horses. You'd be surprised how many upper level show jumpers bounce too- it's just one of those things that takes time and developing a 'seat'.
If ride a horse with impulsion-- you might actually be winded. It's a big movement to absorb, and can be very jarring to the abdomen if you're not used to it. So this is also why I say ride a horse with a short stride or flat movement first. Above all- remember to breathe.
...sorry if this isn't advice per say/any tips..
cantering on, into the familiar and unknown