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 don't be scared away from this site by the rest. The advice you've received thus far is very good.
Having said that, here's some of my advice&questions, some already mentioned:
1. Go see a GOOD lawyer IMMEDIATELY! Don't let her know you've been to see one. Just so you know your options and that you'll be acting from a position of knowledge of your legal situation.
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. Furthermore, if possible, ask a friend to stay with you for a while under some false pretext, to act as a witness against her false allegations.
3. Start documenting your and her care for the kids, if you have them, immediately. I recommend you be as detailed as possible, and I recommend you send the details of the day every day to a good friend of yours or to an alternate e-mail account so you will have time-stamped proof in case of a custody battle. You hopefully won't needed, but if you do, it's better you have it. Don't let her know you're doing this. Once you'll see concrete progress in her actions and reconciliation, you can start stopping the documenting.
4. 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. This is a great thread on selecting a good IC - http://www.survivinginfidelity.com/forums.asp?tid=544948 . Also, read up on EMDR therapy - http://www.survivinginfidelity.com/forums.asp?tid=571247 .
5. Go see your doctor about STD testing. Be aware that if you have sex with her, you risk getting an STD and/or getting her pregnant.
6. C(ontinue to c)onfide in your friends&family about this. You need and deserve their support. Have you told any of them yet of her cheating?
7. Find out the identity of the other men, if there were any. Make sure you check if the boss has a gf/wfie by yourself, don't rely on your ww to tell you the turth. If they have wives/gfs, they deserve to know, just as you deserved to know. Don't trust the other men or your wife that they've been told. Check out this - http://www.survivinginfidelity.com/forums.asp?tid=576398&AP=1&HL= .
8. 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.
9. 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, but that's more suitable for later on and can wait.
10. Go to the "I can relate" subforum here and check out the "Betrayed menz" thread on the first page. Read and post there, it's a great place for betrayed men.
11. If you are giving her another chance, 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 (not the current one that suggested not telling you), 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. 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. Read it on your own at first. The best book for me on infidelity is "Not just friends", look it up in the library.
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" in your library, 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. Tell her that lying will do more damage than any truth will.
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 cheating, or if she'll continue to be unremorseful, 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.
17. A divorce would be hard on you and, if you have (I couldn't see from your posts) kids. When it comes to them, it is, however, much better for the kids to not see you everyday if that means that they won't grow up in a toxic household due to their mom's cheating, disrespect,..., where their dad is being destroyed emotionally due to their mom's actions. What the kids need to grow up emotionally happy, mature and strong young adults is to have at least a part of their lives emotionally healthy and safe (i.e. the days they will spend with you), to have at least one parent (i.e. you) emotionally stable and healthy (and you can't be that if you remain with a cheating, unremorseful spouse) and to have access to a good children's therapist (it's up to you to get them to see a good therapist). Please, consult a good therapist for kids to hear his/her opinion before you decide to stay married to her because of the kids.
18. You DON'T HAVE TO DECIDE RIGHT NOW whether to give her another chance or not, whether to reconcile. 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.
19. Also, check out these two threads, I try to read them regularly - http://www.survivinginfidelity.com/forums.asp?tid=568211&AP=21&HL= and http://www.survivinginfidelity.com/forums.asp?tid=514479&AP=1&HL= .
20. Most of all, keep talking to us, the more you talk to us, the more we can help you and support you!
Best wishes