My horse was very girthy when I got him- he'd try to bite, kick, and squirm around. From what I can tell, he got that way because the person I bought him from would cinch up his saddles very quickly, and he only had him for about 7 months! It took a lot of time, but he is now mostly over it. I made sure to tighten the girth very slowly, one hole at a time, alternating sides, and giving him a small treat each time he stood quietly. He'd also get an elbow in the mouth if he tried biting, or a smack with a whip if he tried kicking. (And, of course, no treat if he got at all grabby.)
I did also discuss with my vet if he might have ulcers, and tried a couple supplements to see if there was any difference, but he didn't show any improvement on any of the supplements.
I heard several different theories about girthiness while I was going through it. My saddle fitter (who's also a vet and acupuncturist) said she thought it was because of heel pain- something about the meridian (?) running from the hoof up along the girth line and up the neck. When she examined him, he was sensitive both on the girth line, and at a point on the neck near the poll. She put a needle in the 'ting' point of each foot and he stopped being girthy while they were in
, so there very well be some truth to that. My horse came with pretty poorly done hooves and was a little sensitive when tested with a hoof tester. His feet are
better now, even though they still have a little ways to go before I'm happy with them.
The other theory I heard was that horses that are girthy don't lead well. This was from a natural horsemanship trainer who I respect. This is another possibility- my horse is a bit lazy and sometimes I felt like I was constantly tugging on him to follow faster. He's better about this now, but is still a little slow about it sometimes.