DONT write the letter, and i'll tell you why. Cause your not detached enough to be able to handle those interactions with her. You will come off as weak and confirm another reason she wants to be away from you. No matter how hard you will try, you will NOT say the right things. Your too emotionally caught up in it still.

STOP trying to let her know how YOUR feeling about the sitch. You really need to focus on this being about you and your kids only right now. By letting her know, your truly only trying to provoke a reaction, it will not the be reaction you were hoping for.

You are either going to stop talking about the M in ANY form and let her have her journey, her guilt, her resolve. OR you are going to be talking to an attorney very soon cause she's going to be filing on you.

BTW you will look more like a fool, if she knows you know and your still trying to get her to stay or come back. That makes you look very weak, and that you'd settle for being her 2nd choice. NOT attractive.