You can try cantering about 5 strides, stop and back up a few strides. Canter a few strides, stop and backup. Keep doing this and soon she'll be anticipating a "stop and backup" and will slow down. It helped me!
You need to sit up and collect your canter they need to be back on their hocks but riding forward all of my horses will learn to shorter canter strides ( slower) and lengthen their canter.
Get 2 poles out with at least 5 -6 strides Sit up and back you don't want to pull the horse but you need a contact he should sit back and wait = shorten canter see if you can try an get in 7-8 strides inbetween the poles its v hard to explain I hope im making sense!