The Queen's Return
Susan couldn’t pinpoint exactly when she stopped believing in Narnia completely, but she knew when she started to stop. It was the term after the trip to America, when she finally became one of Those Girls.

Of course, Lucy's my favorite character in the Narnia stories, but being the oldest in my family I've always understood Susan, and I grieved upon learning that she'd betrayed Narnia herself, and often wondered what happened to her. This fanfic is a very well-done "what if" and one I think Mr. Lewis might approve of.

Thanks to Alexandra for pointing it out.