You said it yourself...she's a halfway decent person and I'm sure her intentions weren't to destroy..it just fell that way.

Definitely talk to your Mom and try to understand, maybe she was desperately lacking something in her marriage that you were never aware of.

Forgive your Mom, she's still your mother. She a mistake. If she didn't want to be with your father maybe she should have figured out the situation before having an affair....but sometimes it don't work that way and they end up living a double life.

I don't blame you for being so hurt, I would be too if it was me in this situation, but be understanding.