Find out about your true boundaries. What is the real limit inside yourself? Is it okay? Is it not okay? Is it okay if...?
The first thing to ask yourself is what are your needs? What are your desires? Is this escapade a betrayal on what you agreed? Do you suffer or are you cool? Does he practice safe sex?
Are you suffering because it happened in your common home? Are you suffering because you lack his attention?
Take one step at a time to protect yourself and gain back your personal territory. If you gave too much take back part of what you gave and see how it feels. If he has the keys of your house for instance ask him to give them back. If you live together, do not tolerate him bringing her home. etc. Before breaking up totally, there are many small steps you can take. Take one step at a time. Build up new boundaries. If it's not enough, the big step might be needed, "bye, bye amigo!" recover your full freedom and independence.
Enjoy!