Bridling my horse?

My horse lets me walk up to him with a bridle, and reach my arm over his head with the top part of the bridle, but when its time to slide the bit in his mouth he simply refuses. He will throw up his head, twist it around, and sometimes even back up a few steps. He is a tall horse(about 15.6 hh) and i'm only 4 ft 5 in. some of my horse-riding friends have suggested to use a mounting block to stand on to get more control, but i'm not sure if its safe. Is it safe or should i try something else?