Girthiness is caused by many factors which include saddle fit.  If you know the saddle fit is resolved than there may be an issue with learned behaviour, or there may be something else going on with the horse's anatomy such as ulcers or old skeletal injury.  

It's important that girthy horses are girthed slowly and it can help to lunge them first with a slightly loose girth and then tighten it. If a horse tends to turn to bite, negative and positive reinforcement can help control the behaviour, ie licorice for not biting and a tap behind the lower front legs when biting to distract the horse and take the focus off the game.