I'm going to break up with my girlfriend for secretly keeping in contact with an ex after saying she wouldn't
Why do you have an issue with your girlfriend talking to her ex?
He's not just her an ex but he's a fuck friend.
If you're sure your partner and her ex are still seeing each other it's fine to break up with her. It was wrong of her to keep a secret from you, anyway.
You're right, I'm going to let her know how she made me feel and break up with her.
It's completely reasonable for you to tell her that she's hurt you, and that you no longer want to see her. You don't have to stay with someone who has been cheating on you.