for me, lots of no stirrup work helped!
To compensate for the horse's movement your pelvis needs to rotate forward and upward, to do that you need strong abdominal muscles and it helps to try not to be "with" the horse's movement but slightly "before" it, try to be faster than your horse 😉
Use your ab muscles, whilst relaxing in the saddle. First try taking your stirrups away while someone's lunging you, and later maybe try without your reins but with stirrups. And if you have a comfortable horse to ride, bareback is so fun and it'll really help because you don't have much of a choice but to sit.
No stirrup work will be very beneficial. Also don't ba afraid to keep a hand in the pommel and pull your seat into the saddle while sitting
Let your core be your shock absorber. Feel the horse move through your hips and go with the horses movement
Arch your back and push your shoulders back and really push down in your heels.
I find squeezing my upper inner thigh helps but this night might not work as well on a really big horse! Hope this helps a little and if not hope you find the answer!