I second Cynical on everything! With Diamond, if you get her going too slow, you'll feel her rocking back and forth almost to the point where it's like she's bucking every step. She isn't, in fact she's trying to be a good girl, she just really can't go that slow. Oatsy can go super slow, while Cowboy can do just about anything lol. But it just depends on the horse.
And that's exactly what I do with DIamond - "You wanna run? Okay! Let's RUN. You wanna stop now? But why? You wanted to run before!!" Make her work, work, work, work, work. Every single time you feel her speed up. After a while of running, give her a break and pat her. Ask for the canter again. If she gives you nice and slow, stop her, pat her. Do this just a few times, and after a while if she gives you the slow canter, stop on that note. Best of luck!!