He's always been the one who rushes to help. As a child, he was the one who always picked up little sister when she fell, the one the younger kids knew would be there when the bullies came. In Afghanistan, lives had depended on his ability to react, to fix it.
He never thought that could turn against him.
He replays it daily, hourly. Any time he cannot force his mind elsewhere.
"Alone is what I have. Alone protects me."
"No, friends protect people."
There had been a reaction from Sherlock, a slight stiffening, a tiny lowering of the eyebrows. John can see it so clearly now. He lives it so often. Why had he not seen it when it mattered? Why had he not seen the wound to fix, the bullied child's cry? He knew they were there, buried beneath that damned aloof mask Sherlock always wore. Why had he not stayed with him?
And the rooftop. It is there every time he closes his eyes. That nerve-shattering moment. It is the same as walking into the tent and seeing the corporal with the gun in his mouth. The moment John knows he can't fix it or heal it, because he's botched his chances one too many times.
Sherlock might have lived. Sherlock might have chosen to stay on this earth, if John had been there just a moment longer. Just a moment of reining in his irritation and hearing him out. His best friend. He ought to have stayed.
"Keep your eyes fixed on me."
Oh, Sherlock. Was there a doubt that he would? The idea haunts him. His best friend. How could he look away? Even knowing what would come? Did Sherlock have so little faith in him at the end?
The well-meaning advice from friends and family is accepted in strained silence. Yes, he's seeing his therapist. No, he's not looking for a new flatmate just yet. No, no, there's nothing anyone can do. He's quite alright. A smile is ordered to appear on his lips. It does. No one cares to see the pain it reveals. He is content with this. He doesn't want others to know what he knows.
If he had stayed, Sherlock would still be alive.