I just wanted you to know that I am thinking of you today.
I remember reading in one of my MANY self-help books that the one who files for a divorce often feels an initial sense of freedom and elation, but they will usually go through an emotional transition about 6-8 months later. If they begin a new relationship right away, this can prevent grieving or delay it... which only makes it more difficult and painful in the long run. I do not think you go through the grieving process unless you spend some time alone. You do not get a real feel for the pain you need to work through. You need that time to learn who you are.
As far as D13 goes, when divorce enters the picture... the children are ALWAYS affected. Kids do not ask for the divorce and seldom benefit from it. You can make the best of a bad situation by continuing to put D13 first in as many ways as you can. By protecting D13's exposure to your anger and disappointment, she will do much better. If you continue to keep D13 out of the middle of your situation, you will be taking positive steps to minimize long-term harm to her. I still think back to my parents' divorce as a stressful and unhappy time that I will never forget.
Is it possible that your X has confided in D13 and has pressured her into maintaining the M as a secret? IMO, children are much more aware of their parents' relationship than most people would like to believe. D13 may not suspect that her mother is capable of such immoral or deceitful behavior (the affair w/ OM)... However, D13 seems to be a very smart girl and I am sure the discovery of the affair blew the vision of her mother being a "perfect parent" off her pedestal. I am sure that it is extremely hard for D13 to have to form some sort of a R with the OM who basically "broke up" her parents' marriage. Unfortunately, your X's choices will have a strong impact on D13.
DMF, you are allowed some "blue days". And, you have to realize that it is okay not to be 100% all of the time when you are going through the midst of this great upheaval. There was a part of me that expected when my divorce was finally over that the pain and misery would end, and that I would be able to pick up and get on with my life. At times, the end of the D has meant there are some more painful and difficult times of transition that are just beginning. I have experienced feelings of being angry and bewildered at the very time I thought I would be finished with such emotions. I once read that divorce can be described as an 'amputation'... "The ghostly presence of the former spouse may haunt you longer after his/her physical presence is gone. In the same way amputees often feel pain or other sensations in a phantom limb, and the aftermath of D brings a multitude of new emotions and responsibilities that may at times seem overwhelming."
DMF, please know that you are in my thoughts and prayers.
I am here for you if you want to talk just as you have always been there for me... Your friend, -KIM