Once I ruled out any pain issues I would try this. Slowly remove the bridle one baby step at a time. Such as one ear, then the other, then slowly start sliding the bridle down the nose. What I am looking for is the point that the horse starts to react and throw her head up. The minute she starts to react, put the bridle back on and take a step back. She knows that you are the one that takes it off and she needs to learn that if she does not stand still, it does not come off. Keep doing it until she lets you take off the bridle without a fight. If this is just a bad habit or a reaction to something that happened in the past, it may take a while to correct, but have patience and don't get frustrated.
You don't have to be crazy to work in a barn, they'll train you!