I'm numb...this all feels so surreal...

My husband and I separated five weeks ago...He had become withdrawn, distant, non-affectionate, angry, sullen...the list goes on. Still, it was a shock when he told me that he planned to move out. Since then, I have gone through every emotion, while trying to keep up my strength and courage. I gave my husband the distance he said he wanted, and I didn't push for all the reasons that he chose to leave. I have been in counseling for three years (not all marriage-related), and together we had been in marriage counseling previously. In addition, my husband sought one-on-one therapy to work through some serious and painful childhood abuse issues...Once a few weeks had passed, my husband had begun to open up a little bit and share some of his feelings. I had hope that we could reconcile, but was still terrified of how things might turn out. At one point, a few weeks ago, we had lunch together and talked some more. He said that he was ready to come home, but I said I wasn't ready. I felt like we BOTH had things to work on to make ourselves healthier. He agreed, but our time with our teenage daughter and together increased. (mostly on the weekends, as we both work full-time). Last Tuesday was our 20th anniversary, and he took me out to dinner. And, although we did have some serious discussion, I felt more positive than I had in a long time. In these past few weeks, my husband became more affectionate, telling me he loves me many time a day, etc. Then, on this past Wednesday, my husband was supposed to have gone to therapy, but lied when I asked him how it went. (Just as a "general" question - not wanting specific details). He came over last night when I asked if we could talk...He stayed the night and left early in the morning because he had to work. I received a text from him saying that he had forgotten his wallet, and could I either meet him somewhere or bring it to him...I'm not one to do this normally, but I felt some sort of gut instinct to look through things. (He had already denied having an affair previously). I found a list of his passwords and looked up his Facebook account. There I discovered a conversation with a women I don't know. The conversation edged on being inappropriate, with the girl asking my husband if he wanted to "cross the line" and either talk or do more sexually...and my husband said yes. After all he has put me and my daughter through, and after all of the compassion and understanding I tried to show him, I just don't understand...why he would lie and say that he loved me and missed me, while also talking to someone else? When I confronted him today, he got very, very angry and said that he wanted a divorce...something he denied wanting over and over again only recently. I would love to hear a man's perspective on what might have happened...P.S. He has had a few recent health scares involving his heart (which turned out ok), and also experienced the sudden death of a close co-worker/friend and learned that his Dad has kidney failure. Depression? (took meds for three weeks and said he couldn't feel anything)...Mid-life crisis...? Please send some clarity and hope my way...