Good luck today. I know it won't be easy or pleasant but I hope this at least gets you out of this relationship that has started to cause you so much pain. Today won't end the pain but it will at least end the painful limbo you've been in since finding out.

Sometimes in these moments the kids get lost. It sounds like you're set with plans for your son and and I'm hoping you plan to send a message to your step-daughter who you've said you care for and is an innocent in her mother's betrayal. Something simple like "Your mother and I are getting divorced but I want you to know you're amazing and can always contact me if you want to and your mother approves." This is going to come out of left field for both kids and be devastating. Your step-daughter will be dealing with losing one of her two homes and probably blaming her mother for that. It might be really helpful for her to know that you don't hate her along with her mom. Your son will have a lot to deal with also but it sounds like you've prepared to deal with that with your ex.

Also, from what you've posted it sounds like your STBXW doesn't complain about you to her AP, actually defends you to him, and that she still acts loving to you face to face. You said you're searching for what you did wrong to make her do this to you but it sounds more like this affair is about her and what is broken with her then about you or your marriage. She probably thought she could have her affair and you'd never know. Not that it matters at this point but I hope she gives you honest answers as to why she's done this. Hell, for your sake, I hope she HAS answers as to why she's done this.