Lily knows she's going to die soon, even with the Fidelius Charm in effect, even though she knows Sirius would sooner die than give away their secret.
She knows they can't go on like this forever, raising Harry behind the locked door of a house that no one can enter. They're going to run out of time, James and Harry and she, and they're going to die. She can feel it. She doesn't know when it's coming, but it's coming fast.
They're going to remember her as a legend, you know, legendary Lily. She thinks back to her Hogwarts credentials-Head Girl, valedictorian, Slug Club; every teacher's favorite student, every student's friend. And what did she do with it? Married young, never worked, spent most of her time in the Order of the Phoenix pregnant or nursing and unable to fight. Lily used to think she was going to be somebody, used to dream of being the next Potions Master or working in International Magical Cooperation; hell, even when she realized that working with Dumbledore's people would be a full-time commitment, she thought it would be more than this, more than bed rest and maternity leave and feeling like a failure.
Lily wasn't supposed to be a failure. Lily was supposed to be Wizarding Britain's breakout success-she was Hogwarts's, wasn't she? At Hogwarts, she used to be somebody; she used to be every girl's best friend, every boy's wet dream, only that wasn't enough for her, was it, because the more best friends you have the less each one seems to love you, and the more boys who want you the less they seem to see you. Severus used to see her, but that was over before it started.
James sees her, and she loves him, she really really does, but it makes her sick that he loves her back. James sees her, but Lily doesn't see him, can't see past the boy he was at sixteen, brash and arrogant and insufferable and the best-hearted person she knew, the only person she couldn't live without. He's different now. There's a war going on out there, and he's reckless and charming as ever, but there's a worry in his eyes these days when he looks at her, a vibe he gives off like he'd die to protect her, and Lily doesn't need protecting, Lily doesn't want protecting. Lily wants James. All Lily ever wanted was James.
But Lily doesn't see James anymore, and it makes her sick with guilt to know how much he loves her.
She spends her days in a little locked house with a baby and a boy, and she's falling apart. She pulled out her cauldron the other night-to try to remember, to try to forget-and she read from the recipe and put in the ingredients and stirred it counter-clockwise like she knew she was supposed to, and the whole thing came out wrong, every time she tried. It shouldn't have been hard.
They're going to remember her as a legend, but she's losing whatever it was that might have made her legendary. Soon she's going to die, and then she'll be immortalized as a fairy tale, one of the greats, and no one will ever know wiser.