Every time she spends a night at the OMs place, she turns me off more and more. But then when I see her and she's acting all sweet and nice it makes me forget her "mistakes".
If that's all it takes for you to change course, then I would suggest that you dig deeper to find out what your core values and "dealbreakers" truly are.
You've stated your supposedly non-negotiable boundary to her ("I will not live in an open marriage") and she basically just took a dump on it.
