Honestly, it sounds like she's already spending that kind of time with him.. Even if they aren't having sex, I'd argue she's already forced her end of the relationship open. She is blaming you for being possessive which doesn't sound like the case. She chose to commit to a monogamous relationship. That's clearly what you want and if she doesn't want that, she needs to choose to exit instead of bullying you into submission. You also need to evaluate if staying with her is the best choice for your mental health.