My name is Ashley and my Mare has been very girthy since I met her four years ago at JWU. I have also helped many horses over come this problem ovef the past few years. I looked up some reasons why she could be, and found that the horse may have ulcers, or back problems.
But also the way I helped her to over come this was I went really slowly when tighting it, but the bezt thing that really helped were treats, or hay…try giving your horse treats, before! You even reach under to grab the girth…the best treat would be something that would keep her mouth occupied for a while; so an apple, hay, carriots. And always tighten it slowly, If your horse transfers to trying to kick you he/she may have something else going on like ulcers or back pain.
I hope this helps, and best of luck.