some "if" scenarios
Finding out the person you feel you can trust the most has actually been dishonest with you for a while is really awful. I'm sorry your situation happened this way.
I'm just thinking out loud (I'm new to this site and to poly), but from what you describe, it sounds like your wife indeed loves you and wasn't deciding to leave you.
What she did was disrespectful to you and your relationship with her... but it wasn't malicious. That is, she didn't do it with intentions to hurt you (I assume).
Let's say that she hadn't done anything with this other woman yet. But rather she went about all of this with utmost respect for your marriage and the love and trust you share, and broached the subject with you about having this outside relationship. I guess I'm thinking you ought to ask yourself if your feelings would be any different if all of this played out the "right" way?
You could have discussed it, the whats and whys and huhs, and your worries and your fears. And you could start to maybe wrap your head around the possibility of it playing out... would it all feel less threatening?
If so, then perhaps you could forgive your wife and start over with some deep conversations about all of this.
Good luck. You're in an emotionally tough spot.