I would focus on you and your son. Any interractions w/her will be an example of the changes you have made (that you considered needed to be made).
I would lead by example.
I would implement some solid guidelines for scheduling of son visitation/flexibility.
I would be pleasant, but only engage in conversations regarding son.
I would cut off her ability to use you as an interim "fix" for conversation/emotional intimacy.
I would absolutely make sure you are protected financially. (debt)
Lastly, I would move on.
She may come back w/real remorse one day... But, Starsky said it best... This isn't a quick fix. You needs to see their changes for more than months... Possibly over a year. Words match actions.
If she doesn't come back w/real remorse, you've already begun a solid foundation for you and son.
Stop feeding her.
Worth repeating and emphasising.
SM, the stuff above bolded, I absolutely did. The financial and scheduling I was on top of, right away.
Did it save my M? I don't know. I'm not with my W, but I'm not D. When I finally stopped talking with my W... ESPECIALLY when I stopped focusing on the OM(s)... she stopped focusing on D...
I then took this past year to make changes in myself, which I'm sure she's noticed... I've learned to be more strong in enforcing my boundaries...
And I figured out what I wanted for my future for me... and with my kids...
I don't know if my W is still "with" an OM... and at this point in time... I don't care... I've realized that I need to move on and I've become (more than) OK with that...
I am not in an open M, we are D in spirit, just not on paper...
But the only thing focusing on the OM is doing for you is keeping YOU stuck...