When you were married, did you and your W exchange vows? Did those vows include monogamy? If yes to both questions, then your wife is cheating and involved in adultery. If the answer is "no" to at least the last question, then you have no ammo here.
I am going to proceed with the assumption you both promised to be faithful to each other, "forsaking all others", and be monogamous with each other. Why else would you be in turmoil? If I’m wrong, please correct me.
My personal view on having a spouse is that you have to share them with other people every day. You share them with family members, friends, their co-workers, church members, if they play a sport, their team members, etc. You get the point. Here’s how you share them:
Physically: Grandkids will want to sit on your spouse’s lap, hug them, kiss them, play games with them, and so on;
Mentally: At work, they must collaborate with others to solve a problem or, generally speaking, to fulfill their task as employee;
Emotionally: They will have family and friends call them and share good news and your spouse will be happy for and with them. Likewise, someone will call with really bad news and your spouse will do their best to comfort them while crying with them;
Spiritually: Others will ask your spouse to pray with them, study with them, or talk about and share spiritual advice.
So, what’s left for you? JUST you? Sexually. In my opinion, when you relinquish that aspect to another, you willingly give up the special and unique link you have with your spouse. When someone else shares that link, you are no different than all the other partners. I know others will 100% disagree with me, but that’s ok! We are all entitled to our own opinions – right or wrong – and this one’s mine. I share it with you because if you and your W DID consent to an open marriage, or you are willing to alter your vows to accommodate this new arrangement, then what you are experiencing should have been expected. If you don’t like what you are now getting, you need to sit down with her and discuss what you are willing to accept and how you want the relationship to proceed from here. If you two don’t agree, then it’s time to part ways.
If you continue in an open marriage, you need to ask yourself some questions;
How will you feel when your W doesn’t come home for several nights?
How will you feel knowing you’ve given her permission to be in another man’s arms? In his bed? Having sex?
How will you feel watching your W get larger as the baby inside her grows over time, knowing this child is not yours?
How will you feel about raising another man’s child for 18+ years? And if this child has a physical or mental handicap which prevents them from living on their own, and you are a caregiver to another man’s child for the remainder of your life?
How do you already feel about her enjoying sex with another man over you?
How do you feel about the risk of her contracting an STI/STD and bringing it home to you?
These are reality questions, and I hope you consider them all.
She has managed to create a complete separation in her mind and wants to focus on us getting better and not talking about what she is doing.
Of course she doesn’t want to talk about it! She knows what she is doing is wrong and she doesn’t want to "face the music"! She doesn’t want to hear that she lied to you about what she was doing and with whom when she wasn’t with you. She doesn’t want to hear that she violated her promise to be faithful to you. She doesn’t want to hear how she put you at risk for a disease. She doesn’t want to hear about how she crushed your spirit and humiliated you in front of her new BF. (I’ll bet you 100 to 1 she has told this guy how much better in bed he is than you, which is why she keeps going back!) She doesn’t want to answer the question from her kids why she is sleeping with another man at night instead of Daddy. Of course she doesn’t want to talk about it!
YOU CAN’T GET BETTER WITHOUT TALKING TO HER ABOUT WHAT SHE IS DOING! IT WON’T WORK!!
Tell her YOU need to talk about it and see if she’s willing to humble herself enough to do so, to help you work through this. See if she’s willing to put you ahead of her "needs". This is true love. If not, you know your place, and right now it’s 2nd. Are you OK with that?
I just read some other responses and your comments:
It seems you are ok with an open M, but:
So yeah, it is an open relationship. If it carries on between them then I would be up for doing similar, just at the moment it isn't something that I want.
What do you want? It sounds like you DON’T want an open M. You need to convey that to her, and tell what you DO want.
And to those who asked, I am actively working on losing weight, getting more in shape etc. I plan to be a lot more selfish and do the things I like and give her space to lust back after me (hopefully!)
That’s the "pick me!" dance. Stop it. You either have her "pick you" now and stop seeing other men – this is a monogamous relationship – or you continue tolerating being treated as 2nd, or at best, equal with other men. Are you OK with this?
And if you go seeking other women and start shagging someone else, what are you going to do when she is pregnant with your child? Will you WW be ok with that? This is a question you both need to address immediately.
Forgive me, OZZY, but all around, this sounds like a very bad idea. But, it’s your life and your M. You need to do you.
I always told her I would prefer that she didn't
This is her being selfish. Since she knew you didn’t want her to commit adultery (unless you agreed to this ahead of time), and she did it anyway, she showed she has little to no concern for your feelings. Sex is very personal and very private. Your desires, feelings, concerns, etc. were of no consequence to her.