The Time Tried our Souls, and Through the Darkness we Overcame (джен)

A fill for this prompt at kinkme_merlin: Merlin goes through a lot of trauma and has gotten very used to hiding it, moving on (or at least appearing to), and coping silently and without anyone noticing, because most of the time no one can know something happened. One day something traumatic happens to him, which everyone knows about, and everyone is waiting for Merlin to snap, only for him to...not. Arthur is caught between worried and suspicious, and sets about to get to the bottom of this.