Fozzie, I can't remember, was there an established practiced religion in the world of the books? I got the impression that only the MCs of each book even knew what Narnia was other than people in the world/dimension/wherever Narnia was supposed to be.
Was Susan raised in a traditional Christian church? Did Aslan ever say to her in plain English that he was actual Jesus, the actual man crucified on the cross that hangs above the altar in the sanctuary she has worshiped in her whole life (he may have, it's been awhile since I read the books)? And then she got booted out of Narnia - as Aslan was well aware that her faith was a bit shaky. Religious practice and rituals sustained regularly on a consistent basis is very different than bouncing back and forth to a fantasy land often against your will, and then being told you're never going back. Without any connection to Narnia and Aslan at all (beyond younger siblings and cousins still visiting, but I get the impression they don't see each other much the older they get), would she have maintained that Aslan is a God and Narnia is real and she must stay devout without regular enforcement? No Christian church demands that, quite the opposite.
Especially considering the entire reason the Pevensie kids found Narnia in the first place is that they were shipped off to the countryside, without their parents, to escape the London Blitz, I could very much see the eldest sister hit a certain age and wonder if it was all some game that turned into some shared hallucination from the stress and terror of the situation and their very young ages.
I feel like she was done dirty.