you dont' have to suck it up and swallow it all at once.
Get someone else , who has not predetermined notion of waht the mare might do, canter her, and canter her, and canter her. Let the mare get some of her ya-yas out. Then, you get on and cnater, and just a few strides, like someone said, then back to trot, then up to canter. Do it twice the first time, call it a day. You'll find it gets easier and easier. And, if you have an arena, then you know she cannot go anywhere and run into an electric fence.