Thanks everyone for your replies...
I wanted to just respond to a couple of things.
First of all, he has known this woman for 6 years. He has always been attracted to her, and they have always shared a connection.
Second, he completely flew off the handle when I told him that I looked at his messages. He was seeing this woman before I knew they were in a physical relationship again. That to me is cheating, not poly.
Thirdly - yes I do have insecurities, and I know that I have signed up for polyamory which consists of loving others. I am fine with him loving others. What made me angry was that I didn't know they were in a physical relationship when these messages were sent. He was supposed to be at work or hanging out with other people during the dates of these messages.
I'm trying...I just cannot handle being lied to. And I don't deserve it. I'm very supportive and allow him to love whomever he wants.