OP, you need to block this person, at least for this first month. You're not in a good space, so you should reach out to all your supports. You don't have to keep her secret, but you aren't required to spill all. Disengage with everything linked to her, it's not worth it and you only feel hurt. Wait a while, if you think in 2 months her boss still should be informed, you can act then. For now, you're deep in the emotions, so if you act now, it might be that you'll regret it because it'll create another major shitstorm you now cannot handle.

Since this is the third time you've been betrayed, it's time you get counseling. You aren't responsible for the cheating, but you might be someone who has a character where you sacrifice easy and too much too fast and thus are an easy prey. If you are doing it for instance because you're afraid to be disliked if you say no, you're apparently doing something to screw yourself and that's something you should fix as it likely messes with you on other areas as well. Counseling can help you figure that out and learn how to set some boundaries.

Take care of you. Look up your local suicide hotline, they can help you with some resources and at least a listening ear.