I don't know what state you are in and you don't have to say if you don't want but if by chance you are in NY then I have to say, from a legal standpoint and the AWFUL divorce laws here, go for the separation.
My H and I have spent nearly 11K in attnys fees (well, he has, lol, I wont be paying a thing) and the court system here for a divorce, especially a contested one (which will happen if either attny files a RJI if an agreement isn't meant by the two parties within 20 days of the initial filing) is nothing short of your worst nightmare. I know because I have been wrapped up in this joke of a court system for 21 months. The BS - the continuances, the adjournments, the pre trial hearings... it is terrible and stressful and we don't have children.
Well, I have her own admissions of her infidelity, but I don't think I'd need it. She wouldn't contest the grounds. She knows I have everything on her, and she wouldn't want such ugliness coming out in open court.
What you describe about D in NY is exactly what I've heard, so I don't think I can go the route of filing for divorce, as much as I might want to. We've almost got our separation agreement finished up. At our last meeting, when the mediator stepped out of the room she looked at me and asked if I wanted to tell the mediator to hold off on the final paperwork. I asked if she wanted to and she said yes, so I said yes too. My hope was stirred up.
A couple days later was when I told her that I only agreed to that assuming we were still vested in our marriage. She nodded and that's when I said I wouldn't live in some crazy open marriage. She said she knows, and that she hates where she is, but she "just needs a little time". The next day I went out of town for a week, and when I came back everyone was sick, and by the time everyone was better I had to go to my marathon, etc, etc. She's used all that to stall. So here I am.
I probably should just stick with the separation. She seems to consider it a very serious step in the ending of our marriage, so maybe it has enough teeth to work.
Back last winter she was full of brass confidence, not caring if I divorced her or separated or whatever. Whenever she would start to waver, she'd talk to OM and he'd get her all riled up again. Now it doesn't seem to be working. Seems like she's on her own now, at least as far as deciding what she wants to do. I don't know what is going on between her and OM any more, but I know it's something, or she wouldn't be hedging like this.