(((peace))) I know this still hurts even after 20 months and the D being filed. Even though you have detached and moved forward in your own life, there are still feelings. I can hear your pain in your words...I am sorry.
I obviously have no clue what your H is doing with this woman BUT my guess is that whatever is going on, he does not consider it a relationship. Did you try to ask your H what his relationship is with this woman or did you just accuse him? Why do you think he has made up his mind? From everything you say, your H is stuck and afraid to commit to anything!!!
I know when these things happen our wounds are reopened and the pain comes flashing back. Just remember that these feelings are short lived plus it is a reminder to keep moving forward and detaching.
Keep taking care of yourself and your kids. You are doing so well moving forward in your life...don't let this set you back!