The memories of that night make him want to vomit, but he can't help remembering. They come flooding back at every opportunity, snatches of moments. Sirius, thundering down the stairs. Sirius, ordering Kreacher to talk to Dumbledore. And Sirius, kissing him before walking out the door, readying himself for Apparition, his lips promising to return though they both knew the odds of that. (Of course he'd sacrifice himself to keep Harry and his friends safe.)
He blames himself, mostly, for not managing to convince him to stay, for not being able to keep him safe and alive. The guilt tries to consume him almost as much as the wolf sometimes manages to. Everything is hollow in the wake of it, and it's all that he can do to keep food down – what little he does eat. He should have gotten Harry and Neville out faster, should have intervened with Bellatrix. It might hurt Sirius' pride, but at least he'd be alive. He'd be here now with that barking laugh and everything wouldn't feel so empty.
Most nights he doesn't sleep, instead wanders London, trying to walk the hollowness out of himself. It's better than drinking – that was Sirius' escape and Remus doubts if he'll ever be able to smell Firewhisky again without facing the memory of Sirius haunting that house, drinking himself into forgetting.
London, too, holds so many memories from that time after Hogwarts but before that Halloween. These are easier memories, ones with a smiling Sirius – devilishly handsome and knowing it – his hair tidy and the ridiculous attempt at a moustache. These he can deal with without feeling nauseous.
Then there are the others, when he closes his eyes, which tear him apart. The mixed fury and terror in Sirius' eyes when they heard about Harry being at the Ministry, a tempest in and of itself. The laugh on his face as Bellatrix's spell caught him in the chest. The fluttering veil as he fell, body so elegant and graceful even in death. And there's nothing that he can do when these memories hit him, except bear them out and not break down.
(So it's no coincidence when – the first full moon after the battle in the Department of Mysteries – the Shrieking Shack is filled with heart-rending howls once more.)