The important word here is "anymore". She used to be able to put it on and take it off, but she has identified with her false self so much, she has become the character she is playing. It's something that happens to con-men, who often can't remember what is a lie and what is true anymore, and to a lesser degree some actors. I know there was an actor, I want to say Gary Oldman, who literally forgot what his real voice was and had to go to therapy to remember what he sounded like without "doing" a voice.
Rachel Dolezal may really believe her lie, but that doesn't give her a pass. And just because she has been successful in deceiving herself doesn't mean she is not still deceiving others.
She is mentally ill, but in our culture, we celebrate that. It's really sad.
She mentioned becoming fascinated by black culture as a child. My guess -- given her dysfunctional family, she was probably escaping in it to the point of losing touch with reality. Children of abusive or dysfunctional homes may often split.