After checking teeth and all that, start training her to drop her head. It's pretty easy. Lay your hand on the crown of her head, right where the halter or bridle rests. Apply steady pressure. Wait. The moment she dips her head (it may take a minute or two, and she might even try to raise her head first), but the MOMENT she dips her head even slightly, release the pressure and rub gently. Wait a second. Ask for a drop again with pressure. The moment she gives it, release the pressure. In a single session, you ought to be able to get her to drop her head to her knees with only a slight pressure on her crown. Be patient, be calm, and mind your timing. You aren't pushing her head down. You can't. You're teaching her to drop her head with the touch of your hand. And it will happen.
Once it's happening, introduce the bridle, and go through the same process. Be careful to not bang her teeth with the bit, but using this process on a basically gentle, sweet horse will work.