I had it for about 3 paces, accidentally, in my lesson last week. But I knew it when it happened! Everything was so much easier, and it fell into place...and of course, as soon as I thought "WHEE! I'M POSTING AT LAST" it all came apart.
I spent the last week trying to keep that weird stretchy feeling in mind, and today in my lesson, the instructor told me to trot the long way down the ring and to either sit or whatever (her words). I squeezed the horse into the trot, and just automagically boinged up and down and up and down. And I got him to stop when and where I wanted to, at the end of the ring. I kept in mind what happened last week, so I waited until me and my steed were at a standstill to say "I think that was a little right!" and the teacher said it wasn't a little right, it was a LOT right, and let's do it again, now.
So we did it up and down the long side of the ring, like, six times.
Next lesson: working on remembering to STEER the horse when posting on a trot. Any time now I will be able to *turn a corner* and keep posting...and I might just want a big gold star or something for that! (and no, I have no idea whether I was on the right diagonal or not - part of this, for me, at least, has been *not* thinking about what I was trying to do or looking at the horse to see where his legs are, and just letting it all happen).