Under saddle, I would work her forward and down. Not all the way to the ground, but with her head just below wither height, and really stretching at the base of her neck. Don't even worry about having her 'on the bit' yet. She also needs to learn to and be allowed to swing her back, and go really forward. Rider her in medium trot and canter everywhere, big sweeping figures like figure of 8's and serpentines will all help her develop muscle, suppleness and a willingness to go forward into the bridle.
Allow your hands to rest just in front of her wither, about 2 inches apart, and don't move them. If you try to see-saw her at all, you will teach her to back off the bit and be nervous of it, rather than to reach into it.
You should be doing this sort of work for a good couple of months before starting to shorten her frame.