How exactly did she show you she could love two at once? Was that before or after your ultimatum? I to suggest finding your own new partner....at least start thinking in those terms. Many of the poly partners encourage it to easy the guilt .... take the pressure off.....loss of libido for spouse...etc.
Having the guy in your house with you there is definitely jumping in the deep end of the pool. You are miles ahead of most first timers.
Good luck. D