This a great suggestion for a young horse, green broke horse, an unbalanced horse, or non-gaited horse. For gaited horses this won’t work. OP is dealing with a specific issue (canter-to-pace instead of canter-to-trot) which is very common in pacing breeds.
OTTSTBs tend to gait or pace when they feel tension and unbalance, and in my experience with gaited breeds the smaller the space you work them, the more likely they are to pace. The more unbalanced they are, the more likely they are to pace… and the more you worry about their head or bodies, the more they are likely to pace. 90% of retraining the OTTSTB isn’t even training it to canter. It’s teaching it that it’s okay to canter in work instead of pace. They are specifically trained to hold their pace or their trot. Now, you are teaching them the opposite of everything they’ve learned for the last 3-10 years, that the inverse is true and you want them to canter and not pace.
To the OP, my question would be, when this horse is romping in the field does he break from a canter to a trot, or a canter to pace? If it is the former, you can work with it – if it is the latter, this may be like pushing stones uphill for you and it may be easier to train the horse to break from a pace to a trot for now.
How are you “resetting” him, when he starts pacing? Do you go back to a walk, or have you installed a clear “cue” to swap from pacing to trot yet? I can’t speak for all STBs and their trainers, but have you ever seen an STB warm up before they work on the track? They usually trot a few laps, then the driver cues them to pace and the real work begins. So most have the cue already installed, but it is for driving, not for riding. You have to teach them the cue is for riding too.
With my OTTSB (pacer), he will come down into a pace from a canter if the former canter is unbalanced, he is too tense, or he is too fast. The canter has to be that Goldilocks canter of “just right” for his downward transition to be just right, too.
I’ve found a good exercise for him is to start trotting the long side at a forward medium trot, with the short sides of the ring being reserved for “spongy”, “softer” trot – think “passage” in your seat, but carry the momentum forward. Once he is reliably rebalancing himself on the short side before you ask, that’s when you introduce cantering. I ask for canter in the first half of the short side from that “spongey” trot, canter down the long side and circle at B (or E), then carry the canter to the opposite corner and ask for trot. This is very similar to the old eventing Beg Novice test A, for a good framework.
The one good thing about OTTSBs is they are incredibly clever and want to work as a rule. However, I’ve found STBs to be the most literal horses I’ve ever met. What they learned driving, doesn’t always translate to what they know undersaddle. With some of them you literally see the light go off in their eyes when they realize you’re just reframing what they already know.
Work on installing a clear cue to drop the pace. Keep the praises lavish and for now, keep the canter-to-trot reserved to a very specific part of the arena, and when they answer what you want, they get praise and a good walk break away from that side of the ring. After a while, once the trot is established, you can work on the elasticity of that trot post-canter which will really strengthen him and make it easier for him going forward.
