I can't recall too much other then this is younger cadet waking alone on a ship alone and he's afraid he's come down with this space madness. He's no longer aware of what time is and he's no idea where in space he is.
The story comes to a close where it appears that he's found the devil in deep space and that he's murdered everyone aboard the ship.
Maybe "The Second Kind of Loneliness" by George R. R. Martin?
The sole inhabitant of a space station spends most of the story wondering why his relief hasn't arrived. Only at the end does he remember that he murdered his relief several months prior for interrupting the solitude he had finally become accustomed to.
Full Text: http://www.lunsfordnet.com/get/pdf/18708.
