What am I doing wrong?

It's really hard to tell with no video but I agree with whats been said about having you're trainer lunge you're horse so you can concentrate on you're seat.
I was damn lucky when I learnt sitting trot because it just kind of clicked straight away, posting to a trot was a whole different story though lol.
My suggestions are similar to whats been said, just relax your hips and try to move with your horse. If you do manage to bounce up out of your seat that's okay the key is to stay relaxed, the minute you tense up you'll start bouncing like crazy.
And remember to breath, I use to hold my breath when trying to canter and that makes your body tense up. Goodluck =)