The trick is to not see the monster too. If you assure him that all is well, you're telling him that there is indeed a monster there. It sounds like you have been able to stay seated no problem, so I would recommend two things. First, take advantage of that big walk. A good 1/4 mile before you reach the monster spot, pick up a good, forward, on the bit marching walk. Get there with purpose. When you get close to the spot, do whatever you need to not anticipate his spook. If you tense up, he'll respond to your body language and spook anyway. If and when he does spook, let him have his moment, do not react in anyway, and then proceed on like it never happened. This method works well with most horses. Can't hurt to try.
As for the fast walking, the previous post advice is good. Turn him back as many times as necessary so as not to indulge his ideas of getting home quickly.
You just have to see your distance...you don't have to like it.