She is looking to you as her "father/authority" figure. Keep in mind that she's acting out like a teen and when that happens the spouse becomes the authority figure that they tend to rebel against.

I would just leave her be. The more you try to stop her from seeing the OM, the more she will be determined to do so.

I know it's difficult, but try to keep the focus on you and your family, as you are the only adult in the house at the moment.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.