Well I have to say I'm shocked. To me, posting is the easiest thing in all of riding. It took me about 5 minutes to learn to post and I thought everyone was the same! It is just a 1-2 beat after all. Diagonals are easy too, even after a 10 year break from riding I could still tell immediately when I was on the wrong diagonal. I don't have to look at the shoulders, I can feel it.
Sitting trot, now that's a different story. It took me about six months to stop bouncing up and down. Even then I still got very tired riding bouncy horses and would eventually start bouncing again. Sitting canter... I'm not even sure I ever learned to do it right. I would always rock back and forth.
Maybe it has to do with weight. I'm very lightweight so being "thrown up" into posting position is very easy for me whereas sitting takes a lot of energy.