I personally find it 100 times more difficult to sit the trot when the horse is hollowed out or stressed. The first thing you should do is get your horse supple and using it's hind end. Then the horses trot will be more balanced and easier to sit.
Now if you are still having troubles try to sit up tall, relax your lower back, look up, and relax your legs. Try to absorb the horses strides with your back.
I hope this helps!
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie