How do you teach a horse to keep a calm collected canter?

I have a 9 year old QH mare. She has lots of spirit and power beneath her strides and i have trouble getting her to slow canter. She has very long stides so whenever i try to keep her canter slow she's off balanced and loses her stride and drops back down to a trot where she's more comfortable, And because her strides are long she can trot just as fast as a canter so i guess that's why she doesn't want to canter slow(deciding to stick with what feels easier to her.) Are there any suggestions on how to encourage her to canter slowly instead of rushing or putting to much power in her strides?