Are your sure your horse's mouth doesn't hurt? Maybe she need a dentist check-up?
If not, try to have a really fixed hand that doesn't move during trot and that follows her neck movement during walk and canter.
Make sure to always maintain a good impulsion so she can carry her herself.
Be very strong in your back when you ask her to lift her head and keep her in front of your legs.
Good luck, hope it helps :)