All good suggestions. I would also recommend asking your vet about the possibility of enteroliths, which could also cause girthiness. Your mare is trying to tell you something, whether it’s physical or emotional. Rule out physical first, which is easier; address emotional once you’re sure it’s not physical. I had a mare that I could not for the life of me figure out why she jumped around when I groomed her. Turned out she was extremely sensitive to static electricity, and I was shocking her. Once I figured it out (she must have thought I was a slow learner!), I would groom her with a damp towel in one hand. She was MUCH happier! But back to a girthy horse, she’s trying to tell you what’s going on. Listen to her.