My Nine Year Old Niece Cries At A Drop Of

My nine year old niece cries at a drop of a hat and says her life is terrible . Oppositional disorder and add been diagnosed has been for her. I have guardianship of her . If she wants to play on the computer and is told no she will silently cry for a long while if you ignore her. If yes then she is very happy and sweet . It's like she is two people depending on yes or no!?

When she cries if you ask her why she can't answer and has an anxiety attack or nothing need to know how to help her