Disclaimer: You know the drill. I own my blatherings, not their subject or setting.
Also, the gorgeous cover image is by wizardeens on deviantART, who can also be found as qutzalcoatl on tumblr.
What happens to Susan is simpler than anyone thinks.
In the time after they tumble out of the wardrobe, the Pevensies cling to their memories. They can talk about Narnia for hours, building fragile rafts of memory on a storm-tossed sea, a life they find themselves second-guessing. Every recollection is a treasure, to be stored and cherished and never, ever lost: they are creating a kind of communal store of memory, somewhere they can go when they feel homesick for the land they once ruled. While it's easier than they'd expected to fall back into the life of English children, they are closer to each other than normal siblings, holding on to each other for dear life: almost co-dependent.
After leaving the Professor's house, and going back to school, it's harder, but they still manage it. Some tiny part of Narnia is held together between them, in cryptic letters with drawings in the margin, in brief but intense meetings – a candle-flame, with four sets of hands shielding it from the wind. They spend as much of the holidays together as they're allowed to (their parents seem rather bemused by this sudden outbreak of family spirit). They have carried Narnia back into England with them, or at least some small part of it, and it's up to them now to keep it alive here.
Humans adapt, to their time, their place, their company. Narnian habits quickly disappear from the Pevensies' behaviour, and while they're not the same children they were before, they blend in well enough. That's precisely why it's so important that they come together as often as possible, that they talk of Narnia in their every letter: sometimes it would be so easy, so terrifyingly easy, to let Narnia disappear from their speech and their conduct altogether. The deeper changes remain, but the simple habits of a Narnian monarch – the habits that renew the soul, always – it would be so easy to lose them. Only when they're together can they clothe themselves in king-selves and queen-selves.
About a year after Susan's final exit from Narnia, her parents offer to take her to America with them for the summer holidays. It's the chance of a lifetime, and she accepts readily enough.
What this means is several weeks – more than a month – without any contact with her siblings. Transatlantic post (when not sent through an embassy) is expensive, unreliable and well-nigh non-existent. Edmund and Lucy have each other; Peter has the professor. Susan has no-one.
Without her siblings, the pressure to be her parents' daughter – and only that – never lets up. She is Susan Pevensie constantly now, a sweet English girl, the Pevensies' pretty daughter who was never any good at bookwork but would make a charming wife. Queen Susan the Gentle, Chatelaine of Cair Paravel, disappears from view. There's none of the earnest Do you remember…? Do you remember…? to remind her now. None of that hidden strength, enough to turn back tides – none of the contemplations of that deep and lovely mind – none of the sheer, unaffected joy that comes only to the wise – none of that is allowed here. The depths of her are off-limits even to herself.
She barely lets herself think of Narnia until she gets home, and then… and then, there are so many things that we believe when we are children. Every make-believe is at least half real, and to believe in them as an adult is so dangerous – requires such great faith – and for such a risk, when perhaps they were never real after all, and isn't it safer to remember them as games? If every childhood miracle were real, we would have to free-fall into a boundless, terrifying future, and we would have to do so alone.
It works so gently and so inevitably that she doesn't realise what is happening to her. It seems only natural that she should see it as a game. Surely that was all it was.
Her siblings look at her with muted sorrow, afterwards. They can remember Queen Susan the Gentle: they remember what a Talking Hare, unusually inarticulate, once said of her – "Being around her is sort of like being near a very deep pool, or in the darkest parts of the forest on a summer day – all sort of still and cool and so beautiful it hurts, but in a very quiet way. She's sort of – she's very deep, Queen Susan is." Still waters ran deep in Queen Susan.
What is left, after Susan returns from America, is only the light playing on the water.