My husband has been dishonest with me about his girlfriend too, so I empathize.
It is horrifying to have someone withhold the truth from you, for whatever reason, but especially when you seemed to be someone SHE could trust with the ugly truth! That's where we are right now, too. I am devastated by the lies. So much so, that even some of the people here on the forum have gotten fed up with me on my blog, for my circular rants. I am trying, trying, trying. It's so hard to discern what to believe and what not to believe, and when you have a family depending on the stability of your marriage, well, what can you do? If it weren't for the kids, I doubt I would put up with it for another day. I love Sundance but the lies may destroy us. I am a forgiving person, but there comes a time it is not healthy to accept unacceptable behavior. I am sure I will know if and when I have reached the point of no return, and you will know, in your situation, too.
Polyamory requires radical honesty. Maybe some people are just incapable of that. Which is terribly sad, when it could have been so different. It sounds like you and your wife really had something good up until this incident. Who knows, maybe she will see the light and take the risk of being honest. My husband and i had a great talk last night, and he said some things I might believe, but maybe not, I just don't know. I'm tired of sifting through it all and second guessing everything he says. I don't want this to be the rest of my life, but just for today, I'm trying to remain calm and let things unfold as they may. Truth has a way of coming out on its own, eventually. Hang in there, Po. You're not alone.