In Twister's case, I had noted some oily dandruff in the girth area when she first arrived, so I suspected that she may have had some kind of irritation or infection in that region that was not dealt with, leading to discomfort. By the time I was saddling her, there was no easily discernible sign of anything that would cause discomfort (the dandruff was gone, and palpating her back and ribs produced no reaction), so I attributed her reaction to habitual response at this point. Fixing this problem would require patient retraining through desensitization, but if that produced no results, I would have to consider the possibility that she had a physical issue I was unable to detect, at which point she would need to be examined by a veterinarian. Fortunately, I did see some progress pretty quickly. Here is what I did:
As with many things, I planned to use an "advance and retreat" method of training. This means that you move forward with what you are doing only to the point where the horse tenses or starts to worry, then you stay at that point until the horse begins to relax, and at that moment, you "release the pressure" (stop what you are doing or go back to neutral), which rewards the horse. Depending on the horse and the situation, you may then want to verbally praise the horse, stroke or scratch the horse, or give the horse a treat. You may also want to step back away from the horse as a reward, as our very presence can be a form of pressure in itself.
To begin, I haltered Twister and draped the doubled end of the lead rope over the crook of my left arm. I chose not to tie her for a couple of reasons: 1) a cinchy horse may pull back if tied, and then you can have a wreck or create an issue with tying the horse in the future; and 2) allowing Twister to feel more free to move around if she felt she needed to would help me to understand how she was feeling about what I was doing, and I needed to know that in order to help her overcome her conviction that being girthed up is a bad thing.
The first step the desensitization process was just touching the girth area and massaging it with my right hand. This did not bother her, and in fact, she seemed to enjoy it. I told her she was a good girl and stepped away for a few moments, giving her some physical and mental space in which to process what had just happened without feeling much pressure from my presence. I repeated this step a couple more times to help reinforce the idea that me just reaching underneath her is nothing to worry about.
I then moved on to picking up the girth with my right hand, being careful to stay aware of her head in case she tried to turn and nip me. I kept my left elbow in a position that would allow me to block and discourage any nip attempt by having her "run into" my elbow with her face. As soon as Twister felt the girth touch her, she tensed up, whipped her head around (bumping into my elbow), then started to move around. I did not try to stop her, but instead just moved with her, keeping the girth in the same position of lightly touching her. She took a few steps before stopping, and as soon as she did that, I rewarded her by letting go of the girth and praising her both verbally and with touch, then stepping back a little. When I repeated this step, she once again walked a bit, though she appeared less tense. As soon as she stopped her feet, I repeated my release and praise. The third time, she didn't move at all, so I told her she was a brain surgeon, gave her a couple of her favorite scratches, and stepped away. I was not using treats, but if I had been, this would have been a good moment to give her one. I repeated this step several more times, making sure that she was completely relaxed with it before moving on. When she looked like she was falling asleep, I knew we were good to move forward.
Now it was time to add some pressure of the kind she would feel when the girth was being tightened. Rather than actually do up the buckles (which takes time to undo), I simply pulled down slightly on the billets while pulling upward slightly on the girth. I was quite pleased to see that while she did raise her head and tense her back, she did not move her feet at all or think about nipping. I released the pressure and told her how wonderful she was and scratched her chest. Doing this step several more times told me she was ready for me to do up a buckle.
Talking to her in a calm, reassuring voice, I did up one buckle a few holes, then undid it right away when she didn't react negatively. I did it up again, and this time did up the second buckle as well. The girth would not have been tight enough for riding at that point, but it was tight enough to keep the saddle from turning over, so we went for a little walk. Allowing a horse to walk at this point not only helps them relax mentally, but it will also encourage them to release physical tension in their back and neck. I was looking for a level or lower head carriage, not a high, "inverted" one. She seemed quite relaxed at the walk, so we stopped and I undid the girth, giving her lots of praise and scratches.
Through progressing with this process, it was not long before I could do up the girth as tight as I would want it for riding, and Twister was no longer worried about it. However, I knew this was not a "magic fix": next time, we would go through the same gentle steps, but I expected them to go much more quickly with less need for repetition. In time, this method would build her confidence to the point that I would be able to simply girth her up like a normal horse, though I will always try to be extra considerate of her sensitivity and avoid tightening the girth too quickly. It is not a bad idea with any horse to leave the girth just a hole or two loose while tacking up, then do your final tightening just before you get on.