My new horse was a bit girthy when I first got him around six weeks ago, and he was really underweight (like, refugee). He's gotten so
much better weight wise over the last weeks, and the girthyness is all but gone. I think his issue was a combo of greenness, inexperience, and ribbiness.
I would continue to experiment with different materials in girths, maybe he's just "picky," like some horses are with bits. A gel padded girth (I see cinches and dressage girths like this, but I'm sure they're available for an AP or close contact) maybe, or one with elastic on both ends that gives a bit more with his movement. As far as the biting goes, I would set him up, almost dare him, to do it (no crossties or flash
), and "strategically place" my elbow for him to hit as he comes around. It doesn't take long for them to discover that every time they try, they get a bony elbow to the muzzle.
Also check your saddle fit. Since this is an issue that has been getting increasingly worse, I could definitely see a slightly ill-fitting saddle over time making him stiff, or get him anticipating being stiff or in pain, and so resisting girthing.