Hey :)
I read this fanfic a while ago, I think it was posted on fanfiction.net and I hope someone can help me to find it.
The story started with Dean waking up to realise that he is alone and Sam is nowhere to be found so he calls Bobby to ask him where Sam is.
Bobby acts kind of strange and just tells Dean to get to his place.
When Dean arrives there Bobby tells him that Sam committed suicide a long time ago.
Dean doesn't understand anything about this and Bobby tells him to look inside his wallet where he finds the suicide note Sam left back then.
I'm not entirely sure but I think Sam was time travelling to kill himself in the past to prevent worse things from happening, but this could be just my mind mixing the story up with another one I read...
So if anyone can help me finding this fanfic I would be very happy :)