whilst I chose to forgive the affair
What does this mean to you? What does it look like? What does forgiving mean to you?
How does it interact with this?
I can never forgive her for it
I don't quite understand how you're reconciling these two positions internally. By "forgive" , do you just mean letting go of your anger or something else? Never speaking of it again?
will break their hearts as long as she’s getting what she wants. I also believe they’ll have a child together if they do end up together and I worry that my children will be pushed aside for the shiny new baby.
I would have thought that this applies equally to your husband. If he chose to do that to his children, he would be responsible for that and he would deserve your anger.
By deliberately forgiving him and deliberately not forgiving her, you just seem to have transferred all your anger, hurt, and fears onto her shoulders - transforming her into a monster threatening your life.
Which made sense as a protective coping mechanism when you were trying to stay with your husband and needed him to be someone you could unite with and trust again, but now that you're separating it doesn't seem so helpful for her to have been built up into this ogre who threatens everything in your life. Instead of helping you cope it seems to be pulling you apart and preventing you from processing your feelings.
Does part of you feel like it would be a breach of your earlier forgiveness to discuss how this is affecting you?
It's entirely your choice who you forgive, when, and for what. Just be careful that the notion of forgiveness doesn't become something that stops you healing /coping - either because you're holding onto anger or because you feel by forgiving you're now not allowed to have or discuss your emotions and fears.