I just saw a Chris Cox episode that involved problem solving of spooky horses. What he did worked like a charm for my horse. Like yours, he's fine with other horses, but it less confident when out alone. Here's what he recommended. Hacking should be fun and relaxing for both horse and rider, so it should be done on a loose rein. If the rider is all up in the horse's face, the horse has no choice but to worry about why the rider is being so active up there. The rules are: Loose reins, hands forward (so you can get contact of either rein quickly by straightening your arm), calm rider, and never let the horse turn away from the object he's afraid of. What Chris Cox did was ride a spooky horse out into the woods. He had a friend jump out flapping a rain coat which sent the horse flying out of it's skin. He gained control by using his loose reins with hands forward, one at a time, to keep the horse's head facing the guy with the rain coat. He didn't do anything with his body other than sit square and keep the horse facing the object. After a good 3 minutes of his friend flapping the rain coat, the horse finally settled down and he was able to walk him up to it. He said the most important thing to remember is to never pick up both reins at once. It can flip over a horse that is spinning around trying to bolt and also cause a horse to rear since it feels trapped. The big trick of the loose reins was keeping the horse feeling safe by not confining it's head, but still having control of the head using the alternating steering reins. He said it's okay if the horse backs up, but he has to keep facing the object. Only when he calms down do you attempt to walk up to or past the object.
I modified this exercise a little. I know what objects my horse likes to pretend to be afraid of, so I started our hack already alternating my reins, so we were walking in a zigzag pattern rather than straight. It probably looked funny, but my horse proceeded to walk calmly past all the objects he objected to the ride before. It worked fantastic! Hope it helps you.
You just have to see your distance...you don't have to like it.