Marilyn Monroe had something called "borderline personality disorder", which is typified by an extreme fear of intimacy, low self-esteem, an inability to form personal relationships, as well as a belief that you have no "real" identity. Many actors do suffer from this disorder, since the only time they can feel whole is when they are pretending to be someone else. They try on identities in the hopes that one will make them feel right about themselves.
In short, if she seems phony to you, it's only because she feels phony to herself as well.
I'm reading a fascinating book about it (which mentions M. M. in passing, but it isn't about her, it's about the disorder), called "I Hate You - Don't Leave Me". I would really recommend it. Turns out I know other people who have BPD as well. It is actually quite common.
Sorta off the topic, but I thought maybe someone might be interested.