Are you sure the girth is the right fit for the horse? Or does the horse "bloat" when it's girth time? If the horse doesn't bloat but you're having to tighten it that much then it's either much too tight, or too small (ie: also too tight).
Could also be that the horse is quite tall and you are quite short. And I mean a serious size difference, lol. My younger mare is 17.3h, I am 5'1", and even though her girth fits her and she doesn't bloat, I can never get enough leverage to do it up all the way. I do up what I can and wait until we get to the arena where I can stand on something to do it up the rest of the way.