Somebody needs some trust? Have you tried in-hand with her? Not much is stopping you from putting her in a halter or bitless bridle and seeing how she does. Many people find the horse more relaxed, going bitless feels more familiar as this is how they lead. Personally, though, I would put flightier horses in a standing martingale with the halter, enough slack that it would only act on them if their nose was in the dirt or their head was directly in front of the rider's face. What worked best for me was the Australian string trick that Schiller uses; you put just enough pressure on the bridle or halter not to break one horse hair, and keep that pressure there until the horse lowers his head, then you immediately release all pressure. Wait a few seconds and repeat. The horse starts responding quicker and lighter in just one session. I think this is easiest with western horses, because you don't need to be in contact with them all the time, the reins can be quite loose. The horse has to be able to accept the contact if you're doing it English. I go around in warm ups with a looser-than-usual rein, and be very particular about where my hands are and how much rein there is... eventually at one point or other the horse lowers enough to feel my hands, I let him, then release a bit more til he can't go further. My own mare changes each ride with her condition issues, so at one point I had to put my leg on and keep her on a circle with the exact same pressure every second until her head lowered and she calmed a bit, then I would let the reins go very light until she bumped her head up again, then I'd put my leg's bending aids on and put her on the circle again. Pressure can't exceed the amount needed to break a horse hair. Your seat is the greatest ally!