In sitting trot your pelvis needs to rotate forward and upwards to compensate the horses movements, to do that properly you need strong abdominal muscles, and it helps to try and do that "faster" than your horse moves, sounds funny but then you'll be in synch with your horse's movement ☺️
I had a lot of trouble to with the sitting trot but it will get easier, and also some horses are easier to sit trot better on it just depends. But like Julia said remember to grip with your lower leg and not your knees, heels down, relax in your seat, keep your shoulders straight and elbows in. Hope this helped😁
Remember to keep your heels down and sit tall. What I try to do is grip with my lower leg and not with my knee, because if you grip with your knee, your lower leg will move around. Try to sit near the front of the saddle on your crotch rather than seat bones and suction yourself to the saddle with your legs. I know it sounds kinda weird but it really works! Focus on your back, legs, and seat.
My horse's trot is very springy and rather uncomfortable. I show in equitation so sitting the trot is something that I just have to do. My trainer has me ride without stirrups at the sitting trot. I focus on sitting deep in the saddle and going with the motion of the horse.