Here's an update:

I was out of town, told W I wanted to talk to son before bedtime Thursday night. Before I call, I get a call from OM's house (caller ID) and it's my son, telling me about the sleepover with his friends (OM's kids). Talk to him, conversation ends with OM in background saying "OK, that's it - time for bed". Son told me Mom was at school.

Now - this is the first day in months I've been away from him, and instead of being with Mom, he's at Turd's house. Naturally, I have an anxiety attack and call W, she calls me back 10pm after school. We have a not-so-nice but not brutal conversation, I tell her how I feel, and how worried I am about the coming separation from my son. I can tell she's anxious about leaving him, but sort of treats me like it's all about her and I, not him.

I get home last night and she initiates a big R talk - tells me she can't love me anymore, but is just choosing the lesser of the two evils. Not that she doesn't love OM, but it's not a good situation, losing everything and destroying our family. She cried the whole time. I didn't, but did tell her how I understood, how I've learned a lot, how strongly I felt we could work things out if she wanted to try. I validated her every feeling, and meant it. We really connected on SOME level. I say some, but no promises were asked for or made.

Woke up this morning and she's on the phone with OM - and it's like nothing happened last night - I just made her melancholy and regretful, and nothing can be fixed between us, OM would never do what I've done to her, etc.
Did I make a huge mistake in even talking to her? She said that if I'd had a change of heart 3 or 4 months ago, everything would be different.
One of my 180's has been a willingness to listen and share my feelings, and one of the detachment strategies has been to not tell her stuff - they conflict sometimes and backfire.

I'm so confused about what to do now - maybe it's just too late for us. I can't be the slightest bit angry at her for any of this stuff.