Ah, I was seeing one part as a "dead end" since there's two ways to get in, but only one of them lets you get out. it's the spot above the long leftwards cave tunnel with the dropdown onto a small hill. if you come at it from below, what I was trying to do, you can't go out from the top. I guess I'm a bit stubborn, and I was kinda assuming the Nightmare's behavior was a simple "go directly towards you", since that's how it acts whenever it's visible to you, but it just following your path makes more sense to what I observed. Given that, it's not actually a dead end. A hint somewhere in the game like "The nightmare follows where you once were, avoid nearing it at all costs. If one route doesn't work, try another." although perhaps phrased more gracefully, would help with that aspect.
I WILL beat this game someday, when I'm less incredibly busy.