What bit are you riding her in? What where is she at in training? Are you working with a trainer? How old is she?
Without knowing this information, here is what I would suggest. Put a smooth snaffle in her mouth and give her her head. Just work on moving out and not concentrating on rating her speed with the reins. She needs to learn to move into the bit, but first of all if this is ducking behind the bit is ingrained in her she needs to first learn that it is ok to put her nose in front of the vertical. Once she is doing this and you can take up contact with her and she won't go behind the bit, you can begin asking for her face, but be sure to push her up into the bit with your seat and legs, your hands will be there for her to come up against and have to give to, not something that is pulling her face in.
We had a horse come back from a trainer like this and spent an entire winter doing what I outlined above.