Hi Steph, Also she was acting out based her expectation that she needs to be defensive (perhaps out of her own guilt) . Seems to me that she worked herself up during the drive home ... preparing herself for you to blame her for not being there and expected you to be arguementive and decided to beat you to the punch. You caught her totally offguard by your calm demeanor. GOOD WORK!
As you continue to avoid any aggressive behavior (yours or hers) , she will start to feel less of a need to be confrontational.
