If there is an OM, there is nothing you can do. Sorry for the harsh truth, but your W is so focused on her relationship with the OM that she is oblivious to anything else. Reading the Divorce Remedy might help you sort out your own feelings and emotions, but it will provide little to help deal with the OM.
Some people say to confront your W about the OM, but that might cause her to go more "underground". Other people say to let the affair run its course, but that could take months or longer.
I will tell you this: do not snoop. Do not check her phone, her e-mails, her online accounts, Facebook, do not drive by her house to see if the OM is there; this will only hurt you and make her angry if she finds out. She will do what she wants even if every one of her friends and family members are furious with her.
Your best bet is to completely detach from her. Do not mention his name or acknowledge his existence when you talk to her. Be as aloof as possible. Be mysterious; not in a fake contrived way, but share as little information about yourself as possible.
This is going to be very difficult, perhaps the most difficult situation you have ever been through. You will need to exert control over your thoughts and emotions if you want to have any chance to save your marriage, and even if you do everything to the letter, there are no guarantees.
I have been through this and am still going through it. My W had an EA and the OM moved across the country to live with her. She has no remorse, and blames me for the marriage woes that "drove her to this." Don't fall for that if your W tries to sing the same tune. Don't let your thoughts and imagination drag you down. Easier said than done, but it is essential for your self-respect, self-esteem and sanity.
This is not about you. It is about her and her selfish choices. Concentrate on your own well-being and try not to look into the past.
Post here often. It helps.
H 56 W 48 D27,S21 SS25 SS22 Severely autistic M(#2 for both) 9 1/2 yrs. "I've never loved you" 3/7/2011 Separated 8/7/2011 BITS