Hold your hands up wide and high and keep her in a contact and stop her from being so heavy on her front end. This might stop the problem all together
Everything said here has been all good but along with my half halts I like to make sure I'mma sitting back in the saddle a bit and it will help shorten the stride :)
is right, I also use half halts to be sure she understands what I'm asking
Lots of transitions, Trot and canter. If she doesn't do a nice transition, do it again immediately. If you get a decent transition, but then she's still heavy, halt, rein back (straight!) and try it again.
This is get her using her back and haunches and therefor will carry on into the canter and she will be able to use her body properly and that'll steady her up. Never get up her, otherwise she'll become tense and it'll make your job 100x harder. This may take a few weeks to be consistent.