I dont know that there is any way for you to express to her that this is such a bad idea.
Well, tonight went from bad to worse, so it came up.
I got to W's house with D, and she was talking to OM on her computer - Told me something about how he was having more problems with where he lives, or something. I didn't really care, plus I was tired, so I just got D together to go to dinner. Dinner was actually a lot of fun - We laughed, had a good time together, and D was pretty well behaved. We went to Home Depot afterwards so I could fix W's sink in her bathroom. Somewhere along the way a conversation started about how W has made the same mistakes over and over (her last 3 relationships pretty much ended the same way our's did, but she does realize that it's her, not everyone else, that is the problem). She talked a lot about how she felt so unloved and unappreciated throughout all of her relationships. On the plus side she told me that she loved me, that she loved spending time with me, and that she enjoyed us and D doing things together.
We got home, got D to bed, and W was pretty agitated... I fixed her sink, then we ended up watching TV for a while. The conversation went downhill from there. W talked about how hard she tried in our M, how much she really put everything she had into us. I tried to validate and listen, but she was really upset. She asked me to leave, so I got my stuff together and we headed into the kitchen. It started up again, and she got really angry and frustrated.
1) She doesn't feel that anything has changed with me, and that if we got back into our M again we'd repeat the same mistakes
2) W said she really needs to be alone for a while and not be involved with anyone.
It was crappy timing but I brought up the OM thing. I told her I felt it would be confusing and stressful to D to have someone living there, male or female, and that I would prefer for us to talk about how to handle it before it got to that point. W was REALLY upset and said she was waiting for it to come up tonight...
She told me to go, that she would look at filing separation to work out 'our issues related to D' and she asked for the key back to her house (I gave it back to her). She pretty much said we needed to not hang out anymore, and that she was disappointed we couldn't be friends.
I really don't understand why we are having SO MANY problems recently. We went six months without a single argument, and now we're going back and forth each week with something that rubs one of us the wrong way. W told me she is angry at everyone. I guess I need to leave her alone for now. I swear every time things get good, they go downhill so quickly. It's not even two weeks since everything went crazy on Valentine's day.
I realize that I screwed up - I antagonized her, and was expecting an answer from her that I wasn't going to get... I really should have just left after D went to bed, but it's too late for that now.