Again, I am sorry that you are experiencing this, but I am happy you are ready to start detaching.
I know it's easy to say but don't think about OM. He could be helping the poor like mother Theresa, but he does not have the character you do. For one big reason...Anyone that seeks out married woman with kids has issues. Even a married women that is unhappy or recently separated (and obviously on rebound). He is a loser with no moral compass and lack of any true insight. He is very selfish. Time will reveal this. So no need to be jealous. Know your value.
I WOULD STRONGLY ADVISE that you immediatly discuss this with attorney... Is there any laws that ensure daughter is not in contact with OM ? Your wife might be dating him, but your daughter should not have any exposure to him. I would take this very seriously. I don't trust anyone when it comes to children. They have no one else but us to protect them, so this is a boundary that cannot be delayed.
Forgive yourself. Recognize your contributions to your marital demise, but this is about her, so don't feel bad about every emotion you have. Your human. Just keep your dignity going forward.
Me: 42 H: 43 Twins age 5 Physically Separated 7/2015
