Try see-sawing her like pulling one hand back, and then the other like a constant movement, or even draw reins.
NEVER SEE-SAW YOUR HORSES MOUTH!!! AND I REPEAT NEVER!
Im sorry if im being alil upfront here, but I can not handle anyone, treating their horses mouth as a tug-a-war game!! Im totally disagreeing with you, harlee rides horses!! See-sawing is one thing... but using draw reins to try and prevent to much implusion. Draw reins, using them while a horse has a problem with maintaining a balanced rythem, in the trot is just asking for a disaster that could be easly avoided!! Ok everyone, get up off your seat I have a little example to show you what see-sawing in your horses mouth feels like to them. Ok I reckonmending doing it outside, with no obsticles... try to run in a straight line while shaking ur head... its impossible isnt it... that's what it feels like to your horse... a great dressage rider said that to me, when I was young, and I have never forgoten...