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Sherlock has always known that he is emptier than most people – his bones hollow in a vain attempt to let him fly – but he's never felt it. Not until now. End of an era. He's been forgiven – hell, he's been upgraded to best friend, which made absolutely no sense logically, (but oh, how unmeasurably glad and honoured he was) – but there would be no John-and-Sherlock anymore.
No more John dropping anything without notice to make sure Sherlock is safe on a case. (He hoped with all his soul that marriage won't mean no more cases for John outright – his best friend would always be an adrenaline addict after all). No more meeting John unless it was scheduled.
They were supposed to drift apart now, as Mrs. Hudson pointed out. She was being kind in her own way, Sherlock presumed. Giving him warning. He could have done without it. Why couldn't he be left with at least the delusion that John would still come by often, of course he would.
He couldn't do anything about that, but he could make sure that John's marriage would indeed be the happiest day of his best friend's life. Whether it involved folding serviettes compulsively or threatening guests. John's big day – the last day before they got inevitably separated – would go without a hitch. Sherlock would see to that.
And if he felt like someone had taken a blunt spoon and was scraping off all his internal organs away, well that was for him only to know. Maybe he would finally manage to fly after this. Entirely empty, Sherlock. Weightless. (With no John to remind him to eat anymore, that ideal would be closer soon.) Then again, why would he want to fly? Who would care about it? (Probably Mycroft. His brother would find a way to make use of that. Mycroft always used him.)
The hardest part was pretending that all was fine. That he was happy. He tried to. Honestly. He tried so hard to be happy for John. But he had been replaced, soon-to-be useless, and it was simply impossible to be happy about that.
It wasn't fair. They'd needed each other, once. Now, Sherlock needed him still – harder than ever – but John had moved on. Why would he want the detective around in any capacity when there was perfect Mary?
Clever Mary, charming Mary, never-sulking Mary (certainly), Mary who knew how to save lives too (nurse), Mary who could – would – give him the perfect family. The only thing John woulld miss were a white picket fence and a dog, and Sherlock couldn't even be the dog. He was, after all, a bird.
John had long since abandoned the nest that was 221B, leaving it empty and cold. Sherlock would swear that the heating didn't work anymore when John wasn't there – but of course that could be just him.
For the longest time, he didn't realize what it meant. Which could be a blessing in disguise – being ignorant of his own condition – and that Sherlock was praising ignorance should have been hint enough that he wasn't himself anymore. (Though that wasn't true – he was what he was, as always.)
But it's only during the actual marriage, worse, at the following lunch – much too late to do anything about it (not that Sherlock could have done anything about it anyway), when he explained to a load of strangers how much he loved John Watson, that it dawned on him how true that was – and not in a best friend way.
Best friends don't burn with such a devouring passion, he's pretty sure, despite being new to this whole friendship lark (the reason he's misinterpreted things for so long). No, he'd fallen prey to the most basic, humiliating human error, and there was nothing he could do about it.
Worse, there was nothing he would do if he could. Loving John Watson had become integral to his very being – far too long ago, Sherlock could see it now – and no matter how much it hurt him, the only thing Sherlock could do was accept that hard fact. (He'd promised Mycroft that he wouldn't get involved when he was already fallen to hell. Mycroft would sneer so much.)
At least it explained why he'd had such a hard time with the waltz or why it retained such sad undertones, no matter what he did. It wasn't a song of love found, but of love lost. (He had never had a chance to begin with. He needed to remember that.)
Once again, he was tempted to use his wings to hug hilself, but the tuxedo wouldn't allow it. He hated the thing. He hated this place. And not even solving a murder could make the day better. Sholto had lost contact with John. John never mentioned him (though, apparently, to Mary he did). It was like looking into a mirror, and finding the man wishing for death was not a surprise. It was definitely the wrong occasion – they couldn't do that to John – but Sherlock found himself wondering how long Sholto would last. How long he himself would last without John. (No, no, no, don't think like that – must NOT sadden John, ever.)
His role in John's life had ended with that waltz though. (Why did Mary have to be pregnant? Dad John will need to take care of himself for his family's sake – no more danger for him). The best thing for everyone was for Sherlock to quietly slip away from John's life.
He could bring his broken self to the sofa and hug Billy. The skull at least wouldn't pick anyone else over Sherlock. It wouldn't leave him. The detective knew that he should have stuck to the dead. Look at the mess he was in now. He was a wreck. And he shouldn't be. It made no sense. Why did it hurt so much losing something he's never had in the first place?