How do you deal with an explosively spooky horse, that has already been exposed to almost everything?

Sekur - grip for me. And in the mean time either a change to a calming grain or calming supplement. Also have vet out if its really bad, could be an internal issue. For example my trainers horse, 10 years old and the most well trained horse on the property, and well behaved, became incredibly spooky to the point of being dangerous to ride. Turned out he had Lyme . Treated it and he was fine.
What are you feeding?
I cared for a horse like that haha I did a lot of ground exercise, and teaching of trust and would walk her around the property it seemed to help a bit. Hope that helps a little:)