I'm just guessing at this point, but I feel like the whole doppelganger part was another one of his hallucinations, and in truth he actually went back out, repressed his memories, and convinced himself that he was over whatever that happened in the shed.
Only to come back in the future, because whatever the trauma was, it will creep back into him.
So the part where he is trapped in the shed is just a representation of how he sees himself, because he lost a part of himself while experiencing whatever the event was 10 years ago, and the him that's outside is just a shell, unable to move on.
But whatdoiknow itsjustatheory a MANGA THEORY