Maybe you should also post this in the "gaited" section of the forum. In the "horse breeds" section there is also a gaited section.
I have the same issue with my Fox Trotter. Except I sort of consider it a non-issue because she's 18 years old and I just trail ride for pleasure. So when she goes into a rough pace I just slow her up and don't worry about it.
But I have been trying to read up and learn all I can about gaited horses, and from what I understand they are typically in a hollow frame when they pace. So you need to work on a more collected frame. In a gaited horse this doesn't have to be very collected at all. Not like a dressage horse is collected. But at least level to slightly collected. Because to pace they are hollow in their back.
Another way to practice cantering is to canter up hills. This shifts their weight back on their hindquarters and discourages pacing. I get my best canters when I am on a slight incline.
Also, understand that a lot of gaited horses are never really trained to encourage the canter. There is a lot of focus put on the flat walk and intermediate gait and very little on cantering. So it's likely your horse hasn't had much cantering practice and the pace is what comes natural to him.
I tend to ride the flat walk the most often, followed by the fox trot and I do like to canter. But I pick my locations strategically if I can. And if I get the pace I slow down to get back out of it. Sometimes I have had luck pushing the horse forward out of a pace into a canter by squeezing my legs while taking contact with her mouth and rocking my body in a cantering motion. But that doesn't always work and I don't know if it is correct. But it sometimes gets her back into a canter.
Basically if you are pacing, your horse is too strung out and needs just a touch of collection. But I know exactly what you have going on there, mine does it too!