Circles do it for me! Generally, I ask the canter, then as soon as they speed up, immediately, but gently, turn them in a tight circle, but keep them at the canter, then continue on the fence. It'll take some patience and a lot of time, but in time it worked for a young quarter horse I worked with. Same with trotting on the right, just circle her when she speeds up. Hope everything works out for you guys!