Thanks for posting another video. I think your are posting in a less forced manor.
May I ask how tall are you? That saddle looks almost too small for you. When I saw you posting from the rear in the video, you were coming right down almost onto the cantle. You should be touching down lightly right in the middle of the saddle, just touchind down and back up again. I can't see real well in the video but I wonder if the saddle is too small for you.
To be honest, I wouldn't call that horse's trot all that rough. In fact, because he is offering you so little impulsion, you have nothing to post off of. One needs some energy to be able to post off of. At that speed I might just sit the trot. I think some work sitting would be good for you. And if you could ride in a very controlled environment bareback it would help your balance. I can see a lot of wobbling going on and if you watch the video again and focus solely on your hands, you will see them go up and down as you post up and down,. They should stay in the exact same place relative to the horse, no matter whether you are up or down. It take time to be able to do this and you are making progress.
If you were my student, I would have you in a round pen and on a lungline. But you are doing the best that you can and we appreciate that. So, next time try to get a little more energy out of the horse and you will actually find posting to be easier. Good luck.