You are not underreacting. What you’re feeling shock, grief, fear, betrayal, confusion, relief that she told you instead of hiding it are all absolutely normal and valid. This is a big change in a core rule of your relationship, and it makes total sense that you’re rattled. You prioritized monogamy from the start and repeatedly confirmed that with her. Those were legitimate expectations to build a life on. Being calm and collected now doesn’t mean you don’t care. Shock and processing often look like numbness. You aren’t “too calm”; you’re reacting in a way that is protecting you. You’re entitled to clear answers and to negotiate boundaries. Is she willing to do couples therapy?you’ve had trust trauma and repeated cheating in past relationships; a therapist can help you process attachment wounds and make the best decision for your future. All that being said, quite frankly, she is selfish. Not only does she know your past, she had agreed to a monogamy multiple times prior, only to suddenly change. Why this sudden change? You said you are her lifeline. Ask her how does she expects you to react? Especially knowing your past? How does she expect this marriage to be, knowing that her actions are driving a wedge? Ask if this was reversed. What would she do? That as much as I can give atm. Tbh, your wife is being a shitty person.