I posted the following response on this same issue on another forum.
"No horse is taller then the tip of his ears when his nose is touching the ground." Pat Parelli
^^ That is an amazing quote full of truth! Your horse obviously doesn't like something involving the bridle. How many wrinkles do you have in her mouth? Do you have a cavesson, flash, etc? Do you always ride with contact? Do you pull her to stop?
You need to teach her the 'head down' cue. With her halter on, LIGHTLY press down on her poll and then SLOWLY increase the pressure, but don't push down harder! When she makes an attempt to drop her head, even a fraction of an inch, release and rub her. Get this good enough where you can ask her to lower her head all the way to the ground.
The next step is to halter her from your knees This can be VERY frustrating for people because this task really shows the person how unconfident the horse is with them. You HAVE to have patience, and if you feel yourself getting frustrated, just stop and rub your horse. You will be amazed at how haltering from your knees will improve your relationship.
Now I would get her used to you touching her muzzle area. Make sure she is confident, and make it an enjoyable experience for her.
Now, for the bridling....you have to have a solid and consistant 'head down' cue for this to work. Ask her to lower her head, and put the bridle in your right hand. With your left hand, place the bit under her chin. Place your right elbow on her neck and don't let your elbow leave her neck. If she puts her head up, stick with her the best you can and ask her to lower her head again, but don't be forceful. Be calm, polite, and sensitive. Now, place your left thumb inside her mouth to get her to open up. Gently push the bit into her mouth, and at the same time lift up with your right hand to raise the bridle. Make sure you don't clank her teeth. From here you want to fold her left ear over and gently push it through the headstall. Now fold the right ear over and place it through the headstall. This is the polite way to bridle a horse.
To take the bridle off, push the headstall over her right ear, then her left. Hold the bridle in your right hand again, and with your left hand hold the bit and slowly and gently time it to where you can take the bit out of her mouth without clanking it on her teeth.