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Deletion
Sherlock deleted Molly after her death.
All he sees when he thinks of her, is a faceless woman in a white lab coat. All he hears are muffled, contorted tones that change octave if he focuses too hard. The memories he has with her in are unfocused and give him a headache.
Sherlock never spoke of Molly following her death.
John had asked him about it once, in a rare moment of reflection but Sherlock didn't answer. At the time John had let it go, being under the impression that Molly's death had been difficult for the consulting detective to deal with.
Sherlock had left him to this assumption and hinted for a cup of tea.
After she was murdered, Sherlock attempted to file all of Molly away but found he couldn't lock the door away and that she kept slipping between the cracks and into his waking thoughts. In a moment of blind panic at the emotions associated with her, he deleted the room.
He couldn't forgive himself for failing her, but he could forget.
When John learnt about the deletion, he punched Sherlock in the face and didn't talk to him for a month.
Much later, after Moriarty was gone, Sherlock asked John to help him rebuild Molly's room. During the rebuild he found an old file with her laughter in it.
That memory is the one he holds on to when he thinks he might relapse, along with the look on her face when she slapped him all those years ago.
He failed her then. He won't now.
The End.