I would ask her to flex at a standstill, i.e, to bring her head around and touch your boot with her nose. Do this on both sides once or twice, it encourages softness. I generally do it every time I ride, at least once.
It's amazing the difference I can feel if I don't ride my Foxtrotter for a week or two, then take him around the pasture. Generally at first he'll be tough to steer and it feels like your trying to ride a brick with legs. After I ask him to flex, he's much more responsive and lighter, and the 'riders flow' is established, even if he's being an absolute turd that day it's a much better ride.
But be sure to ask for the flex. Don't drag their head around and try to make them hold it there, you'll have one peeved horse :) I normally like to start flex exercises on the ground with some treats. Same principle as under saddle, ask the horse to bring their head around and reward the effort, even if they only get it halfway.
Try to think of it as you trying to learn how to do the splits. You gotta start out at the beginning, and work your way up to your goal. If you try to rush to the end...ouch!
I use the vocal cue 'Flex' with a tap on the side I want them to bend. Then I move up to saying 'Flex' with pressure with a lead rope or long rein. After that, we move up to undersaddle flex cues. Nothing beats a supple horse!
Wait! I'll fix it....