I had a mare like that once. We had her checked out to make sure nothing was physically wrong her and the vet couldn’t find anything. I met a lady at the stables where I used to ride once a week and she came up with the solution. She told me to not tighten the girth all the way, initially. Bring it under and attach it loosely. Walk her around in a small circle, then tighten it a little more, and keep repeating until it was where it should be. She told me she had a mare who had become soured on tightening the girth because her first owner would make it too tight. She said it’s like a belt. Some can tolerate it being tightened all at once, and some are just sensitive to it. I followed her advice, and after a while my mare didn’t mind being cinched up at all. I hope this helps.