For me, when posting to the trot, I had to learn to feel it. People have told me to look down at the outside shoulder, and as that shoulder/leg pushes into the ground, I should rise up. I get all sorts of screwed up following this.
I know when I'm posting to the wrong leg, because I get a twisting motion in my post (as I ride, my body twists to the left by nearly 40 degrees and my legs start swinging). I know when I am posting to the correct leg, I get a double bump when my butt hits the saddle, and if I am riding strong, my body does not fight the straight, forward motion of the rise.
So you have to learn to feel it out. I started taking lunge lessons with my eyes closed so I could ignore everything but the feel of the horse under me to understand how the horse moves under me. But I never actually got a great post down, until I started cantering. I never got a great canter down until I started jumping. I'm very backwards, I know.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.