The 3rd adult that comes into a marriage is not (in my opinion) the one who should carry the blame for the problem marriage. Is it possible that you're telling yourself a story about the woman so you can avoid remaining angry (unforgiving) toward your husband?
As for the make-up chick - I'd feel the same way you do. Why do you have to forgive her? Or, you can forgive her, but still feel angry at her.