I think girthing her slowly will really help. I went through a similar problem. Girth was slightly small, I was very impatient. Problem was she would blow out her stomach in anticipation, making it even worse. I would put it on the first hole on one side then go to the other side, bring up the girth but NOT BUCKLE IT. If she let her air out, I praised her and let go of the girth. I repeated this a couple times then put the girth on the first hole after she blew the air out. Then very very slowly I would tighten it.
I got a little sidetracked, I'm not sure if you'll have this exact problem. But with a girthy horse, always SLOW. Their problems stem from their girth being yanked into place quickly, which hurts.