The reason you tell her is because it's being forthright and honest, which is (hopefully) an implicit agreement you two made to each other when you decided to date. If it wasn't, you need to reevaluate your priorities. Maybe exclusivity is not for you. Maybe you're not ready for the responsibilities of monogamous relationships. You also tell her because you respect her enough as a human being to let her decide if she wants to continue being with you. By denying her that you're making her decision for her. That's not for you to do. She's probably not going to take it well based on the story... And it seems like if you tell her she's going to take it t especially badly. If I were you I would start hunting for a new apartment and be 100% ready to move out. Then tell her.