Susan's story was a tragedy you know. It wasn't that she didn't want to believe. On the contrary she wanted to believe, needed to believe so much it hurt. And it hurt so much that she pretended to forget, because every time she remembered the way the sun shone or the trees danced, every time she believed Narnia was real, it hurt. It hurt knowing that Narnia was real, and beautiful and she was no longer welcome there.
Susan refused to waste her life away waiting for something that will never happen. Narnia abandoned her, why should she remain true to it. Once He had said, "Once a king or queen of Narnia, always a king of queen of Narnia", yet He had lied.
Susan has always been good at making the best of what she has, of adapting to what comes and not complaining. She was a realist, she could not place her faith in has-beens and might-bes. She wasn't Lucy with her unwavering faith in Narnia and Aslan, nor was she Peter with his certainty of what had to be done. Perhaps she was most like Edmund, yet even so he managed to believe where she couldn't.
When the train crash happened, all those carefully constructed barriers broke, and Susan lost herself to Narnia again. But she wasn't there, they'd left without her, and Narnia had embraced them while she was still in dreary, grey London. And she woke up from her dreams of lands she had sworn to leave behind, and the pain was so raw, because they had always done everything together and now she was alone.
She was alone at the funeral, everyone else gone, all swept from her as one. It was just her, the priest and the grave diggers. And the priest spoke meaningless words, and Susan knew they were false, because they weren't in heaven now, they were in Narnia, without her. And then the priest stopped and it was her turn and all she could think of to say was, "My family believed in impossible things, and Peter was strong, and Edmund was wise, and little Lucy, she believed more than we can ever comprehend". And then Susan turned and left, because everything hurt, and she had been left behind.