@freaksho3 best guess, is that she never left after her childhood.That's why the child memories are vivid while the teen memories are not. they're illusions. Worst case scenario, she died that time she got injured and if she can see through the illusion the fox made then she'll pass on.
hmmm I have feeling at the start when she said that she fell down and was injured, what actually happened was that she died and the fox spirit being too late to save her tried to keep her soul on earth with illusions so that she won't leave him and pass on.