is the bit fitted properly? Sometimes if it's too low in their mouth they will chomp. And some horses just like to chew! I don't think a happy mouth will help that too much.
Is she just really sensitive when anything touches her girth area? (i know horse like that and there's not much you can do other then be as slow and gentle about the girth as you can). Have you tried barely buckling the girth on and very slowly and gradually making it tighter and tighter? Or you could try letting him walk a bit before tightening it, walk a bit more, tighten a bit more, etc. How is the thin sensitive skin (by her elbows)? Is it irritated or flaky at all? I know a horse who's extremely girthy because it took his owner years (sadly yes, literally years) to realize that his skin flakes off if she uses a leather girth. He's still girthy but has normal skin with a fleece girth.
Maybe you've tried all these things... but just trying to give you some ideas! :)