8 parts Complete MatureSam just couldn't get the memories out of his head, not anymore. Flashes of his time in hell and what Lucifer did to him are just bubbling below the surface itching to get out, until one memory brings his world crashing down around him. There's only one person Sam could bear to be around, Dean. That is, until he tells Dean what's really going on.
Sam decides the best thing to do is take some time away from it all, away from hunting, away from Dean, away from everything in the hope that it'll clear his head, but by the time he's realised his mistake, he's already gone too far to come back from it.