I have the same fear - flew over Grand Canyon airport last year, but couldn't land as it was too close to the edge of a VERY BIG cliff, that's gotta be nuts!
Recently, I've found 2 situations in the air that have made me apprehensive (sweaty palms etc) - the first was after an extended period of flying "on top" at FL70 I looked down through a hole in 3000' cumulus - that made me quite giddy.
The second is a bit more worrying - I've just bought an aircraft with a canopy rather than windows like a PA28 and I'm feeling quite a bit more exposed with the extra visability; with that comes the anxiety.
I flew it yesterday and didn't really want to go above 3000' and found that I was a more uncomfortable with the light chop that was bouncing me around a bit.
VMC