Just want to note that we don't get what Aslan had said to Peter and Susan during their private conversation in Prince Caspian, before they both left Narnia for the last time. Maybe Aslan had said something similar to what he said to Lucy and Edmund at the end of The Voyage of the Dawn Treader? If he did, I think it would add significant depth to the Problem of Susan.
Because imagine looking for that one little piece of Narnia you can bring to a world that doesn't believe in fairytales and magic --- it's going to be hard. Imagine seeing your siblings, who have a much stronger faith in these things than you do and succeeding in their search, and getting upset because in your mind it doesn't make sense. They're always looking back, always finding ways to return there through nostalgia and reminiscing with other like-minded folks, but you look to the future, to this world, because that is where he has placed you, all of you --- and yet you still can't find him. Imagine turning to nylons and lipsticks and party invitations hoping to find him in someone you'll meet, hoping that you'll get the same feeling of being in Narnia by doing the things you had done there as Queen --- throwing and attending balls (or parties, as what they call it here), connecting with people, being in charge of your own identity. Imagine slowly starting to give up because you still have not found him, believing it when the world says they see fairytales and magic as only childhood imaginations, trying to move on from the feeling, trying to move on from Narnia, because that is what makes sense at the moment. Nylons and lipsticks and party invitations become your escape, not evidence for the one you are trying to find. Imagine losing your entire family, and the things in this world where you have tried to find comfort in are not working, and you slowly start to realize, over the course of your life, that maybe you shouldn't give up now, and that you should start to look elsewhere. Because you are still alive, and it must be so for a reason. If he is not of the things in this world, then he has to be somehwere, and suddenly you start to realize that this is where your siblings have been looking to all along. Not to the past, not to the present, not to the future, but to forever (in Aslan's Country with him). Not to the things of this world, but to the things they cannot comprehend. Like fairytales, magic, Narnia, and Aslan. And then everything starts to finally make sense.
Susan will find her way back to Narnia, because she finally understood what faith is supposed to be --- through her own experience. Not from what the Friends of Narnia believed, but what she has come to believe herself after years of dealing with sorrow and grief and pain but to no avail until she had found him.
In the midst of it all, she found Aslan. She found him. And through him, she has regained everything she has ever left behind in Narnia.
