First of all - Forgiveness is a gift you give yourself.
Pisses people off to hear this sometimes, but it's very very true.
Forgiving HER is necessary because NOT forgiving her is hurting YOU. YOU pay the price for holding on to the resentment. It poisons YOUR life.
Also - you said: "How do you forgive someone who won't admit they made a mistake? "
Maybe in some way it wasn't a mistake.
Hear me out. I agree with you, it was morally despicable and unwise on her part.
But as it pertains to YOU - maybe it's not totally a mistake. Maybe she knew on some level that she wasn't good enough for you. Maybe as far as YOUR life goes, this will turn out to be a blessing in disguise.
I know this probably sounds strange to you. It would have sounded that way to me too when my husband of 24 years left. But now that we are divorced, I am starting to realize how much better my life is. And I think, on some level, he knew I deserved better in my life than what he had left to give me.