Why do women always push away the man that treats them good and long for the man that treats them like trash.

I would do anything for my girlfriend. She tells me I treat her better than anyone ever has. Yet she still talks about how she is still in love with and misses her ex-husband. He cheated on her with another woman for 3 years, and she knew about it the whole time, but it took that long before she finally got up the courage to file for divorce. She tells me she loves me, but I know that if there came a day where he wanted her back she would probably leave me without the slightest bit of hesitation.