You Are My Sunshine

During Sherlock's darkest days after the tragedy at the aquarium and John's fury at him, Sherlock realizes that he wants out. He's lost half the people he loves in a span of just a few minutes, and it's burnt him out completely. So, he writes John a note. A letter explaining everything. A letter telling John that it's the end of the legendary Sherlock Holmes.
(My entry for the fanfic Friday From the Heart contest)