Girthiness is not always as simple as people would make it out to be.. definitely rule out stomach ulcers because they are not only common but when they become more severe can be very painful and can lead to long term nutritional and health issues. Next, it is extremely important to make sure your saddle fits, and that you are using a soft, flexible girth with elastic on both ends. Your horse may not be reacting to pain from the girth directly but could be unhappy about the anticipation of being ridden for whatever reason, be it an ill-fitting saddle, pain or lameness in another part of her body, or maybe there is something going on in her riding/training that is making her uncomfortable that a trainer could help you with. There also can be damaged soft tissue along the breast and sternum, or problems with the sternum itself, but that is much less common. Unfortunately though, sometimes they really are just girthy, as much as people will try to explain away every case, it could be stemming from one bad experience in the horse’s past, or just a nasty habit with no underlying cause — many thoroughbreds are this way. Whatever the case is, you will have to learn to respect it and make girthing up as slow and tolerable as possible. Do not try to discipline her (unless she bites you, then I do let them know that they crossed a line) and do not try to crank it up anyway with the idea that she will “get used to it”, this can lead to your horse folding down to the floor or rearing or flipping over which is not only dangerous but will make the process more stressful and reinforce the behavior. Once she learns that you are not going to crank the girth up quickly/harshly and that you understand and want to avoid causing her discomfort, she may be able to relax more during the process and the behavior may eventually fade away. Good luck and be safe!