I had this issue with my paint when I first got it. Like someone else previously mentioned, I was taught to put pressure on the poll to lower my horses head. Do this without the bridle first. Make him comfortable with it. When you are redy to bridle, it helps to put the horse in its stall so it can't back up from you and get away from the bit. Do the thumb in its mouth and put it in. Wait a moment, let him drop it out. Continue this several times. Once its habit, you wont have to do it in the stall.