Hi Chloe, the best way to stop her from doing it is to find from where the problem is coming. She could be in pain when she has the bridle on, maybe the bridle has some point of pressure that she can't stand or maybe the bit isn't the right for her. Some horses are more sensitive than others, for example mine had the same behavior when I was riding him with a figure 8 bridle, then I bought a bridle with a regular noseband and an anatomic crown piece and the problem was solved !
Hope it helped :)
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