I don't personally think Demi is fake. I do think she is confused. Within the mental illness she suffers from, I don't think she even knows who she is. She is also a drug addict and having done as much coke as she has, it is typical to have at least a touch of split personality. She may seem fake but in my opinion, she is just trying to deal with life the best she can.
I on't think she is happy with who she is or how she looks or even how she acts. She has put so much expectation on herself it is unreal. So, when she comes across as being fake, I know better. She isn't a real person so how could she be fake? She will find herself one day though and then we can see if she seems fake.