I had this problem with one of my horses and that was 1. Because he was out of shape when I bought him and 2. Because he didn't do a lot of arena work it was more endurance for him so he wasn't really fit to canter and he was very awkward. Now I'm not saying that your horse is out of shape or anything but what I did is I got my horse back in shape and whenever he would get to hot I would half halt him up. I feel half halting up is more affective. Hope that helped!
lots of slow work, keep them thinking. you must be relaxed, have a more resistant seat to help slow their movement, don't lean forward. best exercise is to start on the outside of the ring and canter. slowly spiral in until they go the desired speed. the start slowly spiraling back out. if they speed up, go on a tighter circle. or the other way is to run them until they don't want to go fast anymore, and continue forcing them to run. then ask them to slow. eventually they figure out it's nicer and easier to go slow. as for the relaxed that comes from you and softness. lots of transitions to keep the ride interesting and keep them on their toes works great