Disclaimer: Do I look like C.S. Lewis to you? No, I don't, thank you very much.
Susan does not lose faith in Aslan after she leaves Narnia, whatever her siblings may say. It is that she has stopped (consciously) remembering Narnia. You can't miss what you can't remember, she reasons. So, Susan forgets, for a time. She does not remember, so of course it does not hurt as she looks in the mirror and sees the face of a young school girl instead of the visage of a gracefully-aged Narnian Queen. She does not remember, so it why does it hurt to pull on dresses that were not custom-made and that don't whisper and rustle as she walks? She does not remember (she insists), so it does still hurt when she is addressed as "Miss Pevensie" instead of "Queen Susan," or "your Majesty?"
Susan knows (deepdeepdown) that if she lets herself remember, truly and fully, it would hurt too-too much. She would cry out in shock every time she looked in the mirror and saw that young, unfamiliar face. She would look about her feet – quite oddly – every time she did not hear the rustle and whisper of her dresses as she walked. She would not answer when called by her name. And it would hurt. (deepdeepdown Susan knows that she would go – literally – insane if she allowed herself to remember fully. And so, she doesn't.)
Susan does not remember Narnia, but sometimes, Queen Susan the Gentle creeps out, if only for an hour. She raises Susan's head up high, knows what to say and whom to say it to, and has a certain air about her that others find attractive. She has a certain compassion that appears lacking in those around her. Her siblings do not see it, so caught up in memories they are, but the rest of the world does.
Susan does not obsess over lipsticks and nylons as her siblings believe. They are merely her mask, you see. If she cannot be Queen Susan anymore – everyone would think her absolutely mad – she must pretend to be someone else. Thus, she needs a mask. Simple logic, yes? So, Susan hides under her mask. She pretends and preens and has (almost) everyone fooled. The only ones who she cannot fool are Aslan (for He is watching, and she knows it) and herself (even Susan is not that good of an actress).
Susan does not feel shocked when the telephone rings and a somber voice tells her that her entire family is dead, and can she come to identify the bodies? She adjusts her mask and makes her way to the station. Their corpses are not nearly as mangled as those surrounding them, and there is a slight smile on all of their faces, their eyes open wide in identical expressions of awe. They look happy in death, and Susan is envious. The mask cracks a bit.
Susan does not cry at the funeral, which shocks Aunt Alberta and Uncle Harold, who are both red-eyed and in desperate need of new handkerchiefs. They stand with Jill's parents and Digory and Polly's friends, and none of whom know how to handle Susan. They expected that she would be even deeper in mourning than the rest of them, and her calm, accepting appearance baffles them. But Susan know that they have all reached Narnia, or something like it, in the end, and so, she reasons, mourning them will be foolish.
Susan does not sell the Professor's small cottage, which was bequeathed to her as she was the only one left on his will, and so inherits everything. She keeps it, along with her parent's house in London, and lives in neither house.
Susan does not stay in England. There are wounds there that hurt too much, and she would rather focus on healing the suffering of others less fortunate than her. She joins an organization, and volunteers to go to places where there are wounds from the war that are still in need of mending.
Susan does not return to England for the first anniversary of the accident. She ends up in a displacement camp, helping those who lost everything during the war. Susan sees the haunted look in children's eyes as they wake up from nightmares, only to find that they have not ended, and everything is gone, and recognizes it as a look that has passed through her own almost every day for the past nine years. She has become very good at helping ease the pain. She does what she does best, gently administering words of hard-won wisdom, and writes down the words of stories of worlds long-gone, so as to not let others repeat the mistakes of her past. She is the most popular person in the camp. When she realizes the date, she sends a telegram to England. An apple tree is planted by the graves of Narnia's Friends.
Susan does not find love. She finds companionship, however, and the man is a far cry from the princes and kings who once fought wars for her favor. He is a sad man, and they meet in the displacement camp. He is a year and a half older than she is, and just as lonely. He has lost his world, too. Together they talk of loss and families and worlds long gone and the struggle of dealing with the aftermath. They do not marry. It is not love, but comfort that they bring each other, which is sweeter than love, Susan thinks, as she reaches for her cracking mask, on her way to the cemetery. She is on her way to bury yet another friend, another person she loved, who was just short of twenty-eight. On her way out of the cemetery, she lifts her mask to take a bite of an apple plucked off a young tree. It's juice is bittersweet.
Susan does not live long past her twenty-seventh birthday. She is lonely, and calls out to Aslan. She goes back to England, sells her parent's house, and visits the Professor's old cottage. It is dusty, neglected and forgotten. Susan opens the old Wardrobe. It is empty. Her mask cracks in two.
Susan does not realize where she is. She thinks she is in Narnia, but Narnia was never this bright. She sees Aslan, who leads her further up and further in, where her siblings and Eustace and Jill and the Professor and Polly and her parents wait for her. She feels them embrace her, and she finally feels happiness. Her broken mask falls to the side, forgotten and useless.