No-Fly Zone
There's a ten-year gap in Remus's career history that more or less corresponds with the 1980s. It makes him virtually unemployable, because who wants a teacher who's had a ten-year nervous breakdown? There's not exactly a shortage of teachers in the magical world. Those who can't do, teach, and after Voldemort disappears nearly everyone gives up on the doing.
When Dumbledore's owl swoops in with an employment offer Remus is in the dole queue, trying not to meet anyone's eyes. He's shabbily dressed and too thin, but no one could mistake him for anything but what he is: a healthy man in his prime standing in line with the old, the cripples, the Squibs. He has met people he knows here before, and he knows they do not know what he is. They cannot see why he can't find a job.
So Remus has no pride left, no scruples, and he takes the job without asking why. Anything should be better than the life he's living but Hogwarts is worse; Hogwarts is a graveyard peopled by his unquiet dead. Sirius, James, and Peter, the ghosts of what might-have-been, and Harry, the ghost of what is. Harry is a paler version of James, without James's passion or his cruelty, and Remus cannot look at him without thinking of James dead on the floor in the ruins of the little house in Godric's Hollow.
When the Order of the Phoenix is reconvened-when it's been established Voldemort is rising-Remus is actually relieved. It gives him a reason to get up in the morning, to eat, to dress. The Order's re-forming gives him purpose and Sirius's reemergence gives him hope. But he loses Sirius all over again and he sits in the big house on Grimmauld Place with a bottle and a gun and prays for the nerve to pull the trigger. This time, he thinks, this time he'll do it. He can't go back to the way he was.
It's Tonks that saves him, and it's funny but until Remus sees her wearing her own face he doesn't realize how little she looks like Sirius. She's come to tell him Dumbledore wants him for a spy, and he should be angry; this is not something anyone-even Dumbledore-should dare to ask of him. But he's grateful to have a purpose, so grateful he goes to bed with her purely by accident. In the morning she seems strangely unfamiliar to him, the first woman he's been to bed with in years, and nearly young enough to be his daughter.
He leaves her sleeping, without even a note. What is one more thing to feel guilty about? He is doing her a service, whether or not she knows it. And he is better alone, is the truth of it. He should have died with James, should have died with Sirius. He will not take his own life, because he still has the power to hurt people that way. But Dumbledore has sent him to look for death and no one can blame him if he finds it.