Sorry if any of this is repetitive ... but ... as you think about "confronting" your wife ...
First, I suggest you not think of it as a confrontation. Make it a conversation.
And put the focus on what YOUR boundaries are, what YOU can and want to do to protect your values and your feelings. Focus on being true to YOURself and YOUR values, whatever those might be.
Try hard to NOT take on the role of a parent figure to your wife. Don't set up the exchange as "I am telling you to do XYZ". You can't and don't want to control her. You don't want her to accuse you of controlling her.
Don't make it an ultimatum. "If you don't do XYZ, I'm going to do ABC." That is controlling.
You might say something like this: "You have been infatuated with and pursuing Mr. Minor Celebrity for two months, and are pining to sleep with him. That hurts me too much. I want to be with you but cannot while your heart is elsewhere. I won't keep being physical with you while you are in love with someone else."
About her mental health, while I do think she could use some professional help, I don't think she'd consent, and I don't think you can influence her to consent. You could say something like, "You seem to believe you and Mr. Minor Celebrity have a relationship when he's said nothing to indicate he has any such interest in you. The degree to which you believe that worries me, that you're not seeing things clearly." But your W isn't going to think she needs help until/unless she hits rock bottom.
Anyway, I offer the above as a suggestion. I don't know if it's the right thing to do or say. Look into your heart, Lex, and decide what your own values are for a marriage. Good luck.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final