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Such kind of unstable, fluctuating behaviour can be seen in a variety of psychiatric problems, like personality disorders, dissociative identity disorders, etc. A detailed history (including past and personal history) and a comprehensive psychological assessment is required to come to a definitive diagnosis. It's good that she is taking the help of a psychiatrist
. Your level of involvement with her is your personal choice, but it is better to be cautious about getting yourself emotionally involved with her before understanding her completely.
Wish you all the best.
- Dr. Jonas Sundarakumar