Basically women are insane. The ex wants you back not because she thinks you guys can have a successful relationship, but because she is frustrated by her not having a boyfriend any more. That frustration might make her think that you guys should get back together but it's only temporary insanity. Try to explain this to her and preferably without calling her insane, because well chicks don't like to be told that they're insane even though secretly they all realize it.