A horse that suddenly becomes sensitive about the head may be experiencing an ear problem. Ticks in the ears is the most common cause of sudden head shyness. Since she is becoming dangerous, you may need a veterinarian to sedate her before treatment. A mild insecticide will normally solve the problem in a few days. It will take a little longer to get the horse to trust having her head and ears handled again. Fungus in the ears can also occur in some horses. Iodine will usually solve that problem. Have your veterinarian check your horse's ears.
You also mention that the horse resists bridling. If she is experiencing teeth problems, she may be reluctant to accept the bit. From your question, it is not apparent if the horse resists having its head handled or if it has to do with accepting the bit. Perhaps having the horse's teeth checked and floated is in order as well.
