First rule out any pain, have the dentist out and make sure there are no sharp parts in her mouth that cause the bit to hurt her. Then make sure your bridle fits. Check out the bit, does it pinch? Is it too small or too big? Chances are she’s had a bad experience in the past. Do you just pull the bridle off of her or do you let her spit the bit out? I would allow her to associate the bit with positive things! Put the bridle on, then take it off (gently! Don’t yank it out) and give her a treat. Do this many times a day, every time you see her until she associates the bit with getting a treat. Hope this helps!
First make sure the horse's teeth and mouth aren't causing any pain, and second make sure then that the bit and bridle are totally comfortable and fitted correctly and aren't causing pain. Just to cross all that off the list. I'm not sure if you live in a cold area, but always make sure the bit isn't cold, a lot of people don't think if this.
To help break the habit, put a sugar cube and the but on to the palm of you hand when bridling, they will learn that the bit = goodies
The treat way works but I find that some horses learn tricks to get the treat and not the bit. I recently imported a mare that did the same thing and the easiest way is to bring their head to the side, since when they bed their neck they cant hold their head as high.
Take a small treat, let them know you have it and put it under the bit so they want the treat. However they have to accept the bit to get to the treat. I don’t really like this way but I was taught to do it this way, I do it differently now.
It could be a way of her testing you. My sisters horse does that to me cause he thinks he can get away with it. Make sure to keep the hand not holding the bridle firmly on the bridge of his nose. Being quick also helps. When I worked on a yard I used to squeeze on the ear to make them bring it down, but that can be a bit harsh