I'm very sorry you are in this situation I know it feels horrific and unreal. But it's happening, so you need to act .
I'm sorry this is happening to you, but glad you are here, because you're going to get excellent advice.
Please take this advice and implement it. We see far too many BH's who simply will not act. You must act to get out of infidelity, and then you can decide what you're dealing with, assess the situation and make an informed decision about whether to reconcile or divorce.
But you must act.
So here's what action looks like - some call it "shock and awe", but it is standard operating procedure for blowing up adultery and stopping it cold. You've already gotten some of this advice. If you ignore this, push it aside as too extreme or tell yourself your situation is unique you will get less than optimal results and may well land in painful limbo with her:
1. Accept that not only is your WW lying, she's lying about everything. Lies of omission and commission both. The affair has been more intense and widespread, and you've already caught her in lies. The sex has very likely been more than you know.
2. Accept that this is your wife, not a space alien or demon who possessed your wife. This is actually WHO YOUR WIFE IS. People are able to constrain their powerfully toxic neuroses for a time, but eventually the mask slips. This is who you married. It's a part of her.
3. Expose the affair immediately to her immediate family - her parents, siblings, etc. This is absolutely necessary. This will give her no redoubt to retreat to. Eventually her family will circle the wagons around her, but the initial shock will put her off center. Exposing the affair in this way also ensures you start off with truth and accountability at the center.
4. See an attorney as soon as possible to outline for you what divorce looks like, what the law looks like and what filing looks like. Consider very strongly the possibility of filing for divorce immediately (this does not mean you actually have to divorce!).
5. Implement immediately a hard 180 on your wife. Look in the library section of the SI sidebar to learn more about this. Soft 180 is harder to implement. In my opinion, the Hard 180 is easier and less complex especially for a recently betrayed spouse in shock.
6. Stop any version of the pick-me dance. Stop begging, pleading, crying, allowing her to initiate affection. You did well to tell her not to come back if she went to him; now you must mean it. Do not respond to her tears. Later, much later, her tears may be tears of true remorse. For now, they are merely tears for herself.
7. Demand she go NO CONTACT with her affair partner. She must write a letter spelling out no contact. Any breach of no contact should end in immediate filing of divorce.
8. She must immediately read "How to Help Your Spouse Heal from Your Affair" by Linda McDonald and immediately begin implementing the steps outlined therein - any balking or avoidance by your WW is a huge red flag (my WW balked and this should have been a red flag for me, but I didn't know any better; now you do).
9. She must turn over all electronic devices for recovery software, such a smart phone and turn over passwords for her phone account, social media, any form of communication.
10. Run recovery software on her phone. I believe the consensus is that Dr. Fone is the best right now, but others may have more recent information.
11. Go to BestBuy (or the UK equivalent) today or tomorrow and purchase at least two-three VARs. The latest Sony model in the $50-60 price range is what you want. Buy some cheap headphones, clip them off and plug them into the VARs. Get some industrial grade velcro and attach them in her car, somewhere in the house where she puts on makeup in the morning. The 3rd VAR is for you to carry in your pocket at all times.
12. She must plant her butt in a chair over the weekend and begin writing out for you a detailed WRITTEN narrative timeline of the affair without omitting anything important to the truth. You don't need to know every whispered word or every stupid emoji they shared. But you need to know "The truth, the whole truth and nothing but the truth." This should be DETAILED. Many pages in length. A perfunctory response from her is a crimson red flag.
13. Find a qualified polygraph examiner in your area and schedule it for two weeks from now. She should have one week to complete the timeline, one week for you to read and ask additional questions, and then she takes a polygraph to verify the authenticity of the written timeline. The polygraph is not a panacea; it's instead a prerequisite for getting the truth and testing her truthfulness. If she balks, refuses, etc. file for divorce. Drive her to the polygraph and give her an opportunity to come clean with anything she hasn't told you. Once she gives you a parking lot confession, continue with the polygraph anyway.
14. Before COB today, she should have at least scheduled an appointment to get a full STD/STI panel to ensure she has not been exposed. You will need to do the same. If she balks or delays on this, file for divorce.
15. She must resign her position and find a new job. Caveat here: if you are leaning toward divorce, your attorney can best advise you here. You don't want her unemployed and owing her substantial alimony. But if there's to be ANY reconciliation, she will need to find a new job.
16. Find a good betrayal trauma specialist in your area and sign up for IC for yourself as soon as possible. If the specialist works with a partner (they often do) ask if your wife can enroll immediately with that partner for her own IC. This particular speciality is best equipped for holding your wife's feet to the fire. Other counselors aren't. DO NOT acquiesce to any kind of marital or couples counseling. I cannot emphasize this enough. Marital counseling is USELESS in an infidelity situation; much later perhaps. But not for the foreseeable future.
17. You should reject any attempts by her to offload responsibility for the affair onto you. This is called blameshifting. Her shitty choices and transgressive behavior are on HER. 100 percent. She's given you a version of the "unmet needs" speech. It is horseshit and here's why: you also have needs and I bet they weren't always met either. Now especially you have some very clear needs for honesty, faithfulness and so on. She violated all of these needs and has not even met this bare minimum of your needs. You're going to ask yourself soon why you should be staying with someone who can't even scrape together enough remorse or empathy to meet your basic needs.
18. Other standard "bag of tricks" gambits and mind games cheaters use you should reject: trickle truth (the whole truth comes out in drips and drabs which will ensure you hate her eventually), gaslighting (you've already experienced this), rationalizing the affair, minimizations (she didn't enjoy the sex or didn't orgasm or he didn't or it was only one time or it didn't mean anything, etc), rewriting the history of your marriage. These are all lies. You might tell her upfront if she does anything of these things to you again, it will result in immediate divorce.
19. Accept that sex with her is not going to be the same now. She gave her body away and you will feel some level of revulsion for her. Now I also mean this in a complicated way. Let me explain: You may find yourself falling into hysterical bonding with her and it may feel great for a good long while. You will both desire her and be repelled by her. Even when HB is going on, you're going to have mind movies about her affair and that's a real boner killer. Eventually even the intense pleasure of hysterical bonding will wear off over time. Then you'll struggle with feelings of emasculation and shame and self worth. You'll need to wrap your head around these feelings: They aren't true, but you will feel them all the same. You deserve to have intimate physical relations with a woman who hasn't intentionally sullied herself and betrayed you, and you deserve sex that is good, honest, pure and highly pleasurable. You deserve this as a man and as a human. And you can get it with another woman if you've divorced. Just sayin'