I crave that "Oh my god what have I done" from her so bad, i know its unhealthy, but i crave it, like a drug fiend. I try to keep it out of my head, and its so prevalent.
What would your reaction be to her, "OMG, what have I done" moment? Based on the stories from the board, very few LBH's get to witness that moment. I mean, at some point in time you may be able to tell she has changed, but she may never share that precise moment and what it took to bring her to her senses. She may not have the experience. As I have said previously, your W seems to be following a behavior pattern.
I have known IRL a few women who followed a similar pattern and went from man to man. On the outside looking on, I could not see evidence that they learned their wayward/rebellious behavior didn't work well for them. They continued repeating the same pattern. So, who knows if your WW will change or if this will be years of a pattern for her. If I thought there was something you could actively do to affect a positive outcome with her......I would certainly tell you. At this point, I really believe that you need to cut ties (except for S3) and move forward in building a new life. Mourn the loss of that girl you fell in love with, and accept that she is gone. If someday your W wants to reconcile, I really don't think you will see the same girl you saw before M.
Here is how I see you, and maybe it's b/c I used to be a lot the same way. You referred to it as determination, but I see you not letting go of the desire to control. This is driving you nuts b/c you have no control over her. When you finally turn lose of trying to control the situation, then your thoughts will calm down and you will be able to move forward. You will know when you truely let go, b/c your body will feel it. As long as you are trying to rationalize and come up with all sorts of thoughts & feelings you think she has......it is a form of you wanting control her. You feed that desire by thinking of all these ways she must be feeling, etc. Can you see what I am saying? Listen, I know how difficult it is to recognize it in ourselves. We can label it as something else, but the bottom line is us wanting & trying to find some measure of control.
Wanting to send that letter is out of your need to control. You want to do some action to snap her back to her senses. That's control. Sending her the letter is heavy pursuit, which doesn't work with a WW. Don't you think other LBH's have tried it? Don't you know it would have been the first thing we would have suggested......if it worked?
I will be very frank about your letter. Although I believe you were trying to sound as if you take your share of responsibility for the breakdown......as a WW, I think she'll see it more like a con job. You try to sound one way.....but your anger surfaces. And like when you refer to S3, in every place you shout out "OUR SON". Clearly you are trying to relate a message by writing this in all caps each time, but I think it will have an opposite affect on her from what you wanted when writing OUR SON throughout the letter. It shows anger, blame and guilting. Although you may be justified in your feelings, I don't think the tone in the letter will have the results you intended. In fact, the letter could work in preventing her from ever giving you the answers you seek.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!