John hit reset on Duke for me

Yesterday I had a pretty rough ride on Duke.  We had a great warm up (in the draw reins), but when I put him to work (I decided to do transitions), he short circuited.  He froze, bent his neck, then bolted; or backed, including until he hit the wall; he spun on the forehand; and he generally was just in full on freak out mode.  The first time, I was surprised, and hit him with the whip.  No response, so I gave him a good kick and got no response.  But then I saw his eye was all big, and after that, his heart started going fast, so I just tried to keep him calm.  I could do anything on him but halt.  We could walk, make circles (and change their size), trot, canter, and make serpentines on the centerline, but as soon as I tried to halt him, he'd do one of the incredibly crazy behaviors and stop responding to me at all.  It didn't matter if I had loose reins or tight reins (I thought maybe the draw reins got too tight, but from what I could see, they were already loose, and I didn't want them to get baggy and catch him in the leg if he was leaping around).
Anyway.  It freaked me out a little because I don't know what I did to trigger such an extreme reaction.
Thankfully, John was able to fit us in for a lesson tonight.  It worked out miraculously, because - for the first time ever - I got out of my Commission meeting (in Westport!) 45 minutes early.
He was jigging sideways at John's (all we did was walk until John got out), but it wasn't anywhere near as extreme as yesterday, and John said that he probably felt "stuck" and panicked.  He said that he once couldn't get a horse to stop bolting out in the cross country field - he just panicked and then couldn't get calm enough that John could get his leg over the top and would bolt (again) every time John tried to get off.
John said that he's working much rounder, but that means he doesn't feel like he has an escape, if I'm holding him in place, and that he's still struggling with bending to the left.  He said he also could have spooked at something and then just got wound up and couldn't unwind, or that he could have a little tweak somewhere that needs to get worked out, like a pinched nerve.
He gave me two easy things to remember at home:
First, do a reset and back things up a bit, do a halt - walk - trot - canter - trot - walk - halt, not skip a gait.  John watched me ride, then he rode, and then I rode again, and we did just a confidence building easy transition between the gaits, like walk, halt for a few seconds, get praised, walk again.  Without the draw reins and even after John rode him, Duke was chewing on the bit way too much for what was going on.  John said we'll work on that later, but for now, just let him know it's ok to halt.  He didn't think it was a nasty, get-off-me, but a genuine fear response from Duke.  John said when they're nasty, it seems like they plot it, and it generally is only one side, where Duke did it both ways.
Second, if he starts to get tight and panicky, switch to something he knows and that is easy for him, like trotting to the left.  Just change what's going on and give him a chance to relax.
The more nuanced part was to try to get his right side to bend and soften and step underneath him.  My leg was too far back, and the aids worked much better when I put my leg back up next to the girth.  John said to make sure not to squeeze and hold, but squeeze and release with my hands.  This actually felt a bit more like squeeze and then give - just a tiny amount - when he halts.  John said not to worry about him flipping his head up at the halt and the walk for now, and I've almost certainly been squeezing and then trying to hold his head down for the transitions.
We did some of the rein out, up, and back to help him bend his neck.  He braces and tries to just move to the inside to make the circle smaller, but if I patiently stick with it, once he gives in that neck, he relaxes all the way over his topline (is how it feels).  But it is really hard to make him give to the right.
The other big thing was holding my core tight.  John had me try to get Duke to breathe with me - breathe in through my nose and out through my mouth, and it took three breaths for me to loosen my core, but as soon  as I did, Duke also did a big sigh and relaxed.  So I expect, especially as I couldn't fix it last night, even though I was talking to him and petting him, I was probably holding my core tight and stiff like a brick.
It was a good learning opportunity, but it took me by surprise - it was such an extreme reaction for something that I don't know what started it.  The only thing that was different was J and Will in there, and I think he just really doesn't like the sound echoing.
For the lesson, John had me put my outside leg on when he was stepping sideways, but a gentle touch, not a heavy squeeze and not leg completely off.  He also had me half halt with one hand, but gently.  Then we tried every so gently easing him into halt from walk and then going back again.  He moved nice and round with John, but it took him a while to relax when John halted him.  It was interesting to watch, because he was nervous and moving his head all around but then would start to flick his ears to listen for John, and eventually breathe, then John would pet his neck and walk him again and start over.  John pointed out he is still a really young guy.
We did bending to the inside work at both trot and canter, and after the work, did the halt -walk transitions a few more times.  By then, he wasn't stepping sideways anymore, but he was still pretty tense about it.
It'll be interesting to see how he is tomorrow, and I see John again on Saturday.  But it shook my confidence - well, it kept me humble just when I was starting to feel like I was getting the hang of things - and it made me afraid how easy it will be to ruin him by not knowing what I'm doing, and wishing I could be down there at Caber so I was always around John and not at home, ruining things and messing them up and making progress go so much slower.
The good news is that I stayed calm (after the first smack and kick) and that I tried a whole bunch of things.  I didn't just sit there like a bump on a log not knowing what to do.  I tried things for 20 minutes, then jumped off, hand walked him, and got back on and tried again.  I didn't fix it, like John did within a few minutes, but at least I thought of things to try.  And once again, thank god for John, and being able to ride with him the very next day.