I have a colt in who has a strong flight response. He started bolting when I'd ask for the first few steps and would stiffen up and not respond to a one rein stop. I was at a loss and talking to a trainer staying with us and he said to change the habit. Start by ponying him off another horse for the first few steps and then unhook, or put him in a smaller pen and get his feet moving. I could have kept doing the same thing over and over again (insanity right?), or change the habit. Today when I rode the colt, he didn't bolt. I changed the routine and set myself up for success. It's not giving in or giving up. You have to let go of human thought and create a new routine for the horse.
All horse behavior is habitual. Use your brain, not violence or force, to come to an answer that best benefits the partnership of you and your horse.