To add to what the other posters have offered: IME, the key to sitting the trot is in the state of the horse’s back. If his back is up and “loose,” the rider can sit all day. When the back is tight, as it often/usually is earlier in the ride or if the horse is not that accomplished in his training, sitting the trot means that jarring, stuttering feeling.
So it’s a bit of a chicken or egg: Sitting well can assist the horse in loosening his back, as an accomplished seat can usually cope with a stiff back. And a “loose” back can show the rider how to really sit the trot.
Maybe one approach could be, if you are working on your own: Do suppling work at the walk, and then see if you can, from there, build contact that will give you a smooth, connected transition into the trot. Sit (and enjoy) those nice first trot steps, then transition down to the walk. Repeat. This might give you enough of a sensation to expose you to that “swinging back” feeling, which in turn will help you sit more effectively. And then build from there.
The one other trick is finding good, consistent dressage instruction that will show you how to build forwardness and building connection. Those are the way in to creating a good back.