First, thank you everyone for your suggestions, kind words and information- I really really appreciate it and it made me smile to check and get so much back from my admittedly disjointed explanation of the issue!
To correct something I inadvertently hinted at, this problem seems to happen in both directions only when cantering. I was out at the barn yesterday and lunging him and when I would play out the lunge to ask him to do a working trot or canter, his listening fell apart anyhow, so we went back to VERY basic, w/t transitions with about the length of a standard lead line between he and I. He listened beautifully when he realized we weren't going to c- but the minute it enters his head that we'll canter either direction, he becomes.... frantic? Rushed? I don't know how to describe it. He just stops listening, he loses focus, he rushes, everything becomes fast, faster, fastest-
Specifically, thank you AnalisaParalyzer for your thorough explanation of a method to work him through this. It made so much sense to me when I read it- I suppose that's the problem with being inexperience; It makes sense, but I wouldn't have put that together myself. I DO, as it happens, have an awesome horsewoman as one of my very closest friends and she is willing to do whatever we need to get my boy back to where we want him.
Also thank you to Corporal- I appreciate the well-wishes a lot and thoroughly understand why you feel that way about OTTBs. When I adopted him, we weren't sure he'd even be rideable and to be honest, I was ok with that. He was my buddy and I couldn't stand to see him go to someone else. He will actually put his head down and into his halter when presented with it (I'm short and he might actually still be growing) because we have worked on that in the barn quite a lot. He refuses to be trailered but will go into his stall like a true gentleman if you just give him slack and lift the lead a bit. He's friendly, inquisitive, likes to be paid attention to and doesn't have any stall vices. I would have even said, before November, that we were working very well towards solid responses to voice cues but all that's backslid considerably since I fell.
For the first four months I worked with him, he had no problems at all being cross-tied, standing for grooming and bathing, fussing and everything else. Since we've moved to the new barn, however, he's put on considerable weight (he was pretty underweight when I moved him) and because he's started to feel really good and healthy with regular farrier visits, getting his teeth floated and upgraded feed schedule, I'm starting to really see who he is as an individual, and he's sassy and a bit nervous at times, like he gets stuck in his own head too much.
Thankfully, I have x-rays that prove his knee is fused solid and he's never come up lame, hot or otherwise on the knee in question, so I don't THINK it's pain. If the problem persists with consistent training in the next couple weeks, I'll have the vet do more x-rays and give him a thorough workup to see if they can find something, but I think this is training rather than physical injury (or hope so!).
Sorry to be so wordy. This is my first horse (turned 31 this year) and first riding experience in a very long time and I'm equal parts floating on cloud 9 over it all and completely stumped!