What is the best way other than doing a small circle to break gait from a gallop to canter?

if she continues it i would say get a stronger bit thats wat i had to do with one of mine. made her easier to controll, but u are also just going to need to work with her on it. make sure u have hear attention 100% of the time. so instead of always riding all the way around the arena change direction a lot do circles every now and then and just change it up so she is focusing on u.