I'm glad you're deciding to end things with the mistress.... It probably won't be easy! But as the saying goes.... Youve made your bed, now you have to lay in it...
Just tell her straight up. You dont want a relationship with her. You took the chance with this, if she makes life difficult for you, you'll have to deal with it.
Shitty? Yup. But then again, the whole situation is. You knew that already though
I don't think there is any reason to flame you. I don't agree with cheating, and I dont think others do either. But you're seeking help. Sometimes we all just need somebody to talk about things with. If that happens to be a forum of strangers, so be it.