How do i get my horse to lower her head without being rough on her mouth? Would using a collar work?

My beautiful paint horse used to be a show horse before i bought her and lately when asking her to trot and lope she will not keep her head down which causes me to not be in full control of her. She is a great horse and i just want her to live up to her full potential. Ive done some research and i see some people use a martingale and others use a collar. I just want to know what would help.