Help!

[CWOL]

Hello, I came from another board and would like to have some input into my situation.
My wife and I have been married for 18 years in what I (and our friends) thought was a happy and stable marriage. We have an 11 year old son that we spend most of our free time and energy on. We are financially secure and I earn about 95% of the income while she holds a part-time job for 15 hours a week.
Seventeen years ago, or six months after our marriage, she had an Emotional Affair (as best as I could determine) with her ex-boyfriend who lives on another continent. I stumbled onto dozens of mostly flirtatious emails to each other and I believe they only met once for lunch. After seeing a marriage counselor and reconciling, we recommitted to our marriage and I vowed to be more emotionally supportive and she vowed to stop communicated with the OM. We waited six more years to have our boy, after I established the relationship was going to last and that we were happy together, which I believed we were.
Fast forward to November 2015: An argument aroused my suspicions again. I discovered that she has been corresponding with the OM again. In fact, I looked in her phone and found that it goes back for at least 9 years! None of the emails were explicitly sexual but there was a lot of innuendo, and that he wanted to hold her “until she’s 90” and that they cared for each other. There were several selfies headshots sent by both parties. The OM had just had his second divorce in March this year and she was complaining to him about me. It looks like they may have only physically met once for lunch a year ago. However, she was planning her separation from me and he was supportive and planning a visit later with her. He promised her his support and for her to be brave.
I confronted her without showing the actual email evidence, and asked her why the betrayal of her promise to me!?! Of course, she denied there was any affair since the OM is so geographically far from us. She gave the standard lines, that we had grown apart over 18 years, especially the past year, and that I’ve been cold and distant and would never change, the standard ILYBIDLYAM speech. She demanded that we separate. Naturally, I stupidly started groveling and asking for forgiveness and said I would change my ways. Of course these were all rejected.
I went to my Mother-in-Law and showed her the emails. She was ashamed of her daughter and pushed her back to me, and wanted us to reconcile, knowing what happened 17 years ago. However, about a month after I told MIL, my WW was able to flip her. She secretly funded her $6K for a divorce attorney and then gave her funds to move out and get an apartment. During this time WW was pretending to work on our marriage with me, and I was trying to be a better husband. However, I sensed something wasn't right and went through her stuff and found the legal retainer.
Now she is about to move out and split my son's custody 50/50. I am reading the advice on this board which encourages Detachment rather than Engagement. What advice would you give to me? It seems like a lot of the Detachment were unsuccessful in getting their marriage back on track. But that may just come with the territory?