Lesson on my core

Today's lesson with Mike had the theme of "core". What's weird is that I have a very difficult time feeling this when I'm riding.
At first, there were a couple other riders, so we started out near the scary end (and looking back - for months I couldn't even ride down there!), and worked a bit on smoothing out the transitions. I hint, hint, hint (3 warnings) and then ask for the transition, and at first we worked on keeping him on the bit - thinking overbent and expecting him to pop up and asking him not to. Willig improved rapidly on this!
So then next, Mike started looking at my core - I have consistently (the whole time he and Shannon have been teaching me) curl up and throw away my contact at the transitions. What was fascinating about this was that it seems to be almost the same as the way Shannon is telling me to come to a jump. Mike described it as playing tug of war - you don't pull with your biceps and you don't curl around your stomach - you create this solid wall with your hip and then lean into it.
So then we worked on the canter transitions using my core, and the very last one was absolute perfection. I lifted him up, up, up and then we light-as-a-feather went into trot.
I am supposed to keep working without stirrups and now consistently work on sitting the trot when I do a down transition from canter.
And my heels - the horror. They are a nightmare - the second I stop thinking about them, they spring right back up. Other than at walk, they're almost always, at best, flat. So Mike KEPT pointing it out - to my chagrin - and I just have to keep thinking about it.
Finally, we also worked on my whole lower leg position - thinking about a thrust down and back (I think of it like a frog's legs look jumping), but it's more of a pushing into the saddle with your leg - all of this just to keep my thigh from riding up on the block.
We also started with some standing in the stirrups - to work on keeping the heels down (which is really toes up).
So these are a lot of tiny adjustments which take a lot of thought and concentration, and are just going to take time to turn into new habits. But man - that one transition? It will all be worth it.
It also motivated me, combined with a picture of my fat arm at a race last weekend, to dig out my exercise books and videos and get serious about working on my hips (which HURT when I try to put my legs back) and my core and my upper body strength. I've been procrastinating on that too long now.