Hi. I'm sorry you're faced with this, but it's great you found this site. You'll get lots of advice here, mostly good, take what you want and ignore the rest. The advice you've received thus far is very good.
Having said that, here's some of my advice&questions:
1. It's great you've been to see a lawyer, that's one of the most important things to do immediately!
2. Go buy a VAR (voice activated recorder) or a GoPro thingy immediately and use it to record all your interactions with her. She might try to falsely accuse you of domestic violence to the police. Better safe than sorry, and there have been betrayed husbands here who were hit with false DV charges. I know you probably think she wouldn't do that, but considering her cheating you don't know her as well as you did. As all of us who were betrayed thought we knew our partners.
3. Set up individual counselling for yourself IMMEDIATELY! You need professional help in dealing with this level of betrayal&shock. Find someone with experience in infidelity and/or PTSD. That was paramount in bearing the pain and healing myself. That will help you both short-term and long-term.
4. Go see your doctor about STD testing. Be aware that if you have sex with her in a moment of weakness, you risk getting an STD and/or getting her pregnant.
5. Confide in your friends&family about this. You need and deserve their support. Have you told any of them yet of her cheating?
6. I too agree with others who are advocating you tell the OM's wife as soon as possible. Every day that passes is another day stolen from her, another day where's she's at risk of contracting an STD from her cheating husband (who might be screwing other people as well) OR having an STD that she already contracted get further developed because it won't be treated in time. Also, every day gives her husband more time to funnel away joint money, to convince her with some bullshit reason to sign the house over to him, to co-sign a loan for his I don't know what,... Every day another chance for him to get her pregnant to further tie her down to himself. Every day is another day when being silent is being complicit to the betrayal of her husband. The saying goes:"All it takes for evil to triumph is for good men to do nothing". Is you potentially saving a few thousand dollars in the divorce worth you NOT doing the right thing by telling her immediately? Worth your conscience if she gets HIV from him? Please, think strongly about this, talk to your friends and family.
7. Make sure you eat, sleep and exercise as much as possible. If you're having trouble, talk to your doctor or a pharmacist. DO NOT DRINK ALCOHOL. It's way too easy to spiral out of control. I stopped myself quite close to becoming an alcoholic in the aftermath of it all.
8. Google "No more mr. nice guy pdf", it's a great book available online for free that might be a source of great strength and self-esteem for you, as it was for me. Also, google "Codependent no more pdf", also available online for free. Another great book to read on this topic is "Married man sex life primer", I'd recommend you go to the library and get a copy of it.
9. Go to the "I can relate" subforum here and check out the "Betrayed menz" thread on the first page. Read and post there.
10. You DON'T HAVE TO DECIDE RIGHT NOW whether to give her another chance or not. That decision can wait for quite some time (but make sure waiting to make the decision doesn't hurt you legally, financially, physically/health-wise,... -> talk to your lawyer etc.). Remember, even if you start the divorce proceedings, you can stop them. And even if you finish the divorce proceedings, you can still reconcile with her despite divorcing her. If she's truly remorseful, she will be desperate to get you back even if you divorce her. And if she'll be willing to stay without you despite the divorce, it will be a good sign she's doing that not because she wants the financial,..., security of the marriage, but because of you. So again, no need to make quick decision.
Also, like others have said, it's a roller-coaster, being cheated on. It's one of the most shocking and traumatic things you'll ever experience in life. It is perfectly understandble to be dazed and confused from all this, to not know what exactly to do, to feel one thing one moment and another the next moment. As time goes by and you work on healing yourself, things will get considerably better.
What's crucial is that you don't make any really bad decisions in this first stage of being totally mentally fuc-ed up, like getting her pregnant, getting an STD, you cheating on her with someone else and thereby demeaning yourself, you becoming an alocoholic, crashing a car, beating him up and ending in jail etc.
11. If you will be giving her yet another chance (remember, all the times you've confronted her, every day since the first day,..., was another chance to her that she squandered and used against you), she needs to go to IC at least for a month before you two even consider MC together. I recommend YOU find a good IC for HER, one that you will know has a no-nonsense approach to infidelity (knows that cheating is 100% cheater's fault,...) and go to her first session with her to make sure the counsellor knows all about the cheating. Only after that should you try MC with her.
12. For suggestions on how to try to reconcile, google "How to help your spouse heal linda macdonald pdf", it's a great book available online for free that might help you two tremendously.
13. What was her and your childhood/youth like? Please, google "Toxic parents pdf", it's a great book available online for free. Also, check out "Families and how to survive them", it opened my eyes on the topic of childhood issues and how they connect to adult relationships.
14. I think it's important that you tell her that as a condition for you to attempt to reconcile with her is for her to write you a detailed timeline of the affair(s). Upon telling her, notify her you'll want her to undertake a polygraph test afterwards to ascertain you have the full truth.
15. A phrase that I've heard a lot here and with which I agree is that you must be willing to lose the marriage(relationship) to save it. You mustn't think of her as the prize and you mustn't approach this from a position of weakness, of you trying to win her back by being nice etc. You must get therapy, you must get into the mindset that you will be ok even if you divorce and that you will not settle for rugsweeping or anything less than her being fully remorseful and committed to redeeming herself and helping you heal.
16. If she continues to have contact with him despite you demanding she stops, and if you'll want to stop the affair, EXPOSE HER CHEATING to her friends&family and/or SERVE HER WITH DIVORCE PAPERS. Remember, even if you serve her, you can always stop the divorce later if she proves remorseful etc. But it's a good wake-up call for the cheater that very often works.
Most of all, keep talking to us, the more you talk to us, the more we can help you and support you!
Best wishes