Answer

By Your Horse

Natural Horsemanship Expert Charles Wilson advises:

Breaking the habit will take time and determination on your part. Firstly, I would double check his teeth and mouth, as problems deep in the jaw bone or at the very back of the mouth are often missed. If his mouth is fine, the fit of the bridle needs checking, as does the saddle. If you find no problems, there is still every possibility that the habit began with pain or discomfort when he was bitted and ridden as a youngster. There are some horses who never accept metal in their mouths, especially if early learning taught them to fear the process.
Try long-reining him from a halter or a lungeing cavesson. If he goesforward well, try long-reining on the bit. If he stops or even backs up you’ll know it’s the bit that he’s not accepting. If he can accept the bit and walks confidently forward, then the problem is with ridden work.
There may be an element of sheer defiance. Despite all provocation, never retaliate by getting stirred up – this shows the horse you are not a leader to be trusted. Remain calm and be prepared to out wait him – no matter how long it takes.