You know that feeling you get when your mind happens upon a concept so brain-bogglingly hard to deal with that you try to stop thinking about it so as to avoid losing it completely? But then you realize that by thinking this you are only going farther down the path towards thinking about it. And by thinking about that you get even closer. So rather than dwell in that never-ending mucky muck anymore you actually try to contemplate this idea. Then you think "Well, that's it. I've finally done it, I took my mind to the one place I shouldn't have and it fell over the edge. I've lost it, my marbles I mean, well, I didn't lose them as much as obliterate them. That's what your existence is now. Obliterated marbles. Oh well, perhaps now I can find out what exciting things insanity may bring me.". Then you lay there for hours, waiting for some kind of insane delusions or perhaps a nice little psycotic episode. But nothing happens. Then you open your eyes and remember the ramen noodles you were heating in the microwave. What? You don't know that feeling? Oh, nevermind, forget I said anything.
sorry it took so long for me to find this... perhaps its lucky that it did take me so long to see it. because now i know what you're talking about. i think i posted a thread about seeing over the edge and going too far. basically the same thing. it was quite terrifying, but it happened very much as you said, i went quite far, realised there was far further i could have gone, yet feared having gone too fr already. its a dangerous thing, you feel it almost pulling you in, but you realise the danger and go no further... but maybe thats what happens to some people who suffer some forms of mental illnesses... they dont reccognise the danger and go smash their marbles by jumping over the edge.