Here's a random idea I had about the similarity of some of Sherlock's feelings and the Carpenters' songs. So each chapter is prompted by a different song, though I'm not going to try to write the song lyrics or titles into the actual chapter, because that kind of thing annoys me a bit. Hope you like it.

Masquerade

Are we really happy with
This lonely game we play
Looking for the right words to say
Searching but not finding
Understanding anyway
We're lost in this masquerade

Both afraid to say we're just too far away
From being close together from the start
We tried to talk it over
But the words got in the way
We're lost inside this lonely game we play

Thoughts of leaving disappear
Each time I see your eyes
And no matter how hard I try
To understand the reason
Why we carry on this way
And we're lost in this masquerade

...

Sherlock wasn't entirely sure why he and Victor still lived together when their relationship had fizzled out long ago. They weren't compatible with each other, though it had taken them a stupid amount of time to work it out.

He didn't know why they didn't just separate, but he guessed that it might be something to do with the hold Victor had over Sherlock. He wasn't sure what it was exactly, probably something to do with Victor being Sherlock's first, but both Sherlock and Victor knew it was there. Plus, Victor seemed to like having Sherlock around – he was attractive, and always compliant when Victor wanted sex, and generally did as he was told.

Sherlock used to care for him, but now...now he couldn't stand the sight of him; not with the way he'd been used and abused by someone he cared for. He was tired of going round in circles and forever pretending about how he felt.

While Victor was out, Sherlock packed his belongings, wrote a note and left, taking a black cab to Mycroft's house. He didn't like depending on his brother, but he was intelligent enough not to let his pride get in the way of a sensible decision.