Please just assume Susan was born in 1908, I thought so, and wrote the entire story before realising I was wrong. If you want you can think of this as following Left Behind, but you don't have to, it doesn't affect the story either way.
All Wrong
The year was 1969.
Susan looked at the calendar and sighed. She felt old. She looked it too, although not in a bad way. She'd aged well, graceful in all that she did. Her hair was silvery grey, but still long, and not visibly any thinner than it had been 40 years ago. No traces really remained of the colour it had once been, but Susan had never really contemplated dying it, it just wasn't something she would do.
Susan busied herself making a cup of tea for herself. She was alone in the house, she'd never settled down after the death of her siblings. Her eyes clouded over for a second, she was completely lost in thought. Her mind was racing Lucy barefoot across the grass, and their hair was blowing in the breeze, the sky was blue, and she could smell the freshness of the air. Smell the scent of rain, of summer ending and the encroach of autumn. The first leaves were beginning to float off the trees, in fact there was one dancing and twirling dizzyingly past her in the wind right now.
Susan was jolted rudely back to the cramped indoors of her kitchen by the whistling of her kettle. She shook her head in an attempt to clear it, and began to pour the boiling water into the teapot. This was ridiculous, she was 61 years old, and still believing in childhood fantasies. Narnia wasn't real, had never been real. That wasn't a memory, no matter what it felt like, it was just a dream, her imagination running amok. She stubbornly ignored the way her senses protested, the way her feet could feel the grass underfoot, and her face feel the breeze against it. It wasn't real, none of it was real.
Susan poured the tea out of the pot and into her mug. This was wrong, it shouldn't be a mug it should be a teacup, queen's did not use mugs. She shook her head once more. There she was again, imagining things, she had never been a queen (except a beauty queen once, in America, before the crash) and she'd used mugs her entire life. She took the milk out of the fridge and poured some onto her tea, not too much, not too little. She'd perfected the art of making tea, over the 40 years she'd been doing this routine.
Susan carefully picked up her mug, then moved into her living room. She placed it down; not on the wooden side table, that'd leave marks, and switched on the television. She sipped at her tea and watched. When she saw someone step on the moon she knew in her heart that was wrong, then when the earth rose, from that distance a perfect sphere behind the man, she knew it to be wrong all the more. It took her a moment to place it. After all, there was nothing strange about what she could see, nothing impossible, even if it was new. Then it clicked. People shouldn't be able to visit moons or planets or stars, because they weren't balls of gas or rock, they were alive, and so that was wrong. And the world wasn't round, it was flat, so that was wrong. And she shouldn't be on Earth, she should be in Narnia, and this is all wrong.
Susan shook her head again. What was up with her? There was nothing wrong with anything, asides from her, and Narnia didn't exist, it was just a game they used to play. She'd tried to have herself committed once. She'd gone to an institution, but they'd said there was nothing wrong with her, just an overactive imagination. They'd given her drugs to take, but they hadn't helped at all. They were supposed to dim the false memories and allow her to concentrate on life, but instead they'd made Narnia far more vivid and real, and her perspective of the earth had faded, almost as if Narnia was the more real of the two. She wondered if perhaps it was. No she knew it was just imaginary. Her siblings had died in a train crash, not been whisked off by some talking lion. A flash of Peter saying almost condescendingly to someone that "He's not a tame lion".
Susan turned off the television and scooped up her now empty mug. Returning to the kitchen, she began to wash it up. Perhaps Narnia was real. No, she knew it wasn't, it was impossible, logic said it couldn't be. Yet if not, then why did all of this seem so wrong, and Narnia was so right. She was so confused, all she knew for certain was this was all wrong.