So, a lot of people get super up in arms about Lewis saying that Susan stopped believing in Narnia bc she liked boys and makeup, but that is not his point.
Susan was bitter. You would be, too, if you had grown up with respect as a queen and then got pulled back into a young teen's body without any of that respect. "Oh, she's just a child, what does she know?" and so-on and so forth. Then when she gets the chance to go back to the world that she knows, it's 1000 years in the future.
One thousand years. For context, that's like living in Greenland with Eric the Red, and then getting pulled into modern times.
Then, she was once again pulled back to her world and told that she would never be able to return to the place she loves so much.
So, she starts making herself believe that Narnia was all make-believe as a coping mechanism. She tried to grow up faster than she should've so that she could get that respect back that she had in Narnia.
Have you ever been hungry for something, but don't know what, so you nibble on various foods in the kitchen, never finding out what you were craving? That's what happens when you have a hole in your heart that needs to be filled by God - or in Susan's case, Aslan. She turned to the world, 'nibbling' on grown-up things and a grown-up way of life to fill the hole in her heart left by Narnia.
Fast forward, and she's in her early twenties. Her family probably tried to bring her to Christ, as they know that Aslan's name here is Jesus. But now they're dead, and she's alone.
(This next part is heavily based on a fic I read many years ago, so if the author sees this, I just want to say that it was an amazing read)
One day, she sees Lucy's Bible. She starts flipping through it because she remembers how at peace Lucy seemed while reading it, or maybe she swept it off the table in anger, wondering why a loving God would take her family like that.
Either way, it opens to a page with a drawing of a lampost in the margins. A verse is underlined: "Your word is a lamp unto my feet, a light unto my path."
Susan starts from the beginning of the Bible and turns the pages, pausing anytime she sees a drawing. Each one stirs her heart and mind, and her days in Narnia begin returning to her. Finally, she reaches the resurrection story. The drawing? The table split in two. Susan cries as she realizes what her siblings have been trying to tell her all these years, and she believes.
Years down the line, she passes away with a smile on her face as she is reunited with her family in Aslan's country and sees her King face-to-face once again...