Answer

By Your Horse

Natural Horsemanship Expert Charles Wilson says:

There are several things that you need to check. Firstly, there may be a physical problem – a vetting would not necessarily cover things like an ear abscess, or problems in the lower jaw or back teeth. Once all the checks have been done and physical problems ruled out, check the fit of the bridle. Are the bit and the bridle itself large enough for him? There should be enough room for two fingers to slide under the headpiece, throatlashand noseband. Have fewer rather than more wrinkles at the side of his mouth. Is the bit itself mild? A jointed full cheek snaffle is ideal for a young or green horse. Also, because of the number of nerves at the end of the nose, avoid drop, flash or grackle nosebands.
Assuming all of this is fine, we are left with a mental problem to overcome, which may result from pain. It will take time for him to learn that there is nothing to fear from the bridle. For the moment, continue with taking the bridle apart. Try putting it on from the offside, as this can sometimes be different enough for the horse not to react.
You need to train him to bring his head down. With just a headcollar or halter on your horse, stand by him and lay your right hand (if you are on the near side) where the headpiece would lay. Your other hand holds the leadrope loosely. Now slightly pinch your fingers together – you can increase the pressure a little. The instant your horse lowers his head, take your hand away, to reward him. Stroke him, then repeat, until he learns that the slightest pressure on the top of his head is a cue to lower it.
Once he is comfortable with lowering his head, you can simulate putting the bridle on. Always reward him often with a stroke and a reassuring word. It will take time, but it can be done.