Bravo Donna! Keep the focus on what is best for the kids and try to put everything that hurt you emotionally in the past to the dark coner of your mind called knowlege and life experiences.
I agree with the advice given by oldtimer that it is highly unlikely that XH's gf is evil. She and your H were caught up in the moment of new found passion which overwhelmed the day to day routines of married life which required continued work and ambition to keep the love alive. Her actions during that time were distorted because of the new found passion and the guilt of doing something wrong.
It is still no excuse for what she did and I am sure it will be something she and your H will have sorrow, or even regret, in the many years to come. My mom admitted that she had her own demons about leaving my brother and I following her A and subsequent divorce. But it does not mean that they cannot also put such bad things in the corner of the mind and move forward with a good life ahead.
You will get there in time. Your marriage was much longer than mine so it just needs more healing time. It is also most likely harder when it is a friend that betrayed you. You probably need to stop stirring up in your mind the poor qualities as a mother from her past. Look to what she does now. It the kids get along with her, that is going to work out well. I say this because Ed was estranged from all or some of his 4 sons. But he is good with my kids - to the point where they tell me more than I care to hear about how much they love him.
NoCode - You seem to want to stay on that high moral horse and let what happened in the past continue with a future of bitterness towards those that betrayed you. I get your whole forgive but never forget, but forgiveness also allows for locking away the bad things into a dark corner of ones mind as knowlege. Most people should not be comapred as snakes - people can change and not just shed a skin.
No one is saying to be friends with those that did such a terrible thing to break up a family or putting blame on Donna. We just want to see her move beyond the disgust she has for the prior friend who was a contributer to the break up of her family. It is much easier to move forward with kindness than to keep a tense hostile attitude towards those that have betrayed in the past.
I had some huge bitterness and disgust for my XW. It would cause my blood to boil when bumping heads with her and some of it was because of what she did in the past to our family. She also, during the time of hostility, must have seen a counselor because her tone changed (almost like an assignment from the C) and she became quite gracious in our conversations. I think it rubbed off on me as I find that when we both speak now that there is never any conflict. We both seek clarification as to our kids schedule and needs and we even occasionally shoot the breeze. And we dont mention anything of our past relationship and why it failed.
Look for olive branches being extended by X's and respond in kind. It is so much easier to live in peace with X's than one of resentment and anger and constant suspicion.