I know that this story has the word "Moderation" in the title, but I can't remember who it's by! In the fic, one of them leaves the other (Sam leaves Dean, I think) without telling him why and Dean angsts to him in text messages about how much he misses him. First-time fic, definitely over 3000 words... for the life of me I just can't remember the author!

ETA: Found! "Moderation Itself" by sevenfists . Definitely a good read, you should read all her fics if you haven't already. :)