Susan never stopped believing in Narnia. At least, not the way they all thought she did.
For as a young woman in post-war England, she felt the pressures of getting married. All her friends were either engaged or it was all they could talk about.
The question came from relatives and professors and neighbours and the lady at the grocery store.
And all she could think about was how in Narnia, no one expected that of her. Any relationship she had entered and every proposal she had considered were her choice.
She had grown into an adult before, but in a world where she was valued for herself. She was admired for her skills in addition to her beauty. Not just her beauty.
And if your memories of another life are so much better than the one you're living, isn't it easier to forget?
Isn't it easier to throw yourself into life right now and try to get as much happiness out of it as you can?
When Susan forgot about Narnia, it wasn't because she thought it wasn't real, but because it was too real, and she knew she couldn't go back.