Mare Needs to Learn Manners?

Our mare is getting back into work again and doing great. The only problem is that when we start to tighten the girth she comes around to nip whoever is closest. We've tried to cinch her up slowly, doing a little at a time or tightening a bit then putting a bridle on then tightening it more. She's been getting a little pop on the nose, but I'm worried about her becoming head shy. Is there anyone who's had this problem? If you have any advice, training tips, or personal experience please share! Thanks in advance!