All of our horses here on the ranch in the western U.S. are working horses... cow ponies, but they teach as much as they learn. We often often encounter situations, especially when working with livestock in all kinds of terrain, where the trust between rider and horse has to be equally shared. A horse depends on its rider for leadership and senses very easily when the rider tenses up or is frightened. This causes the horse to react in the same way. You must keep in mind that the horse is a flight animal. No matter hundreds if not thousands of years of domestication, the horse is still ready to react to any percieved danger by running away.
So, having said that, I would follow my father's years of horse training and do the following:
1. Lead the horse to the "dangerous" area after placing some really yummy food in a feeder of some kind. I would use rolled oats. Do this frequently over a period of several days...maybe a week or so. Get the horse to associate a pleasurable experience with the "dangerous" area.
2. Then, ride the horse to the same grain. As you approach the area, speak confidently but quietly to the horse. He's depending on you. Don't tense up... don't increase (even unconciously) your breathing rate... don't grasp the reins or lead tighter... any of the things that transmit fear to the horse.
Be consistent and be in charge and I think you'll see a change... Best of luck