I totally feel your pain. I just got my first horse and until recently posting was the bane of my existence. In the last couple of weeks it is coming a lot easier and I'll share a couple of things I have noticed that I think are helping. First, you have to not think about it so hard. This is a huge one, really don't over-think and it becomes easier. Let your horse push you up and you just concentrate on not slamming into his back. For me the easiest way to start feeling this was to have someone lunge him so that I only had to think about my balance.
The other thing lunging helps is that it keeps him moving better. I was amazed at how much easier it was to post and be "with the horse" when he had a nice forward moving trot (which is SO hard to get when you're riding because it is hard to squeeze and steer and try to learn to post all at once!). If you can't have someone lunge you, carry a dressage whip to help with forward motion. My guy just needs to know I have it, I never even use it on him and he is much more forward.
Good luck and stick with it, I know it is frustrating but at some point it all starts clicking and you'll wonder why you had such a hard time with it in the first place!