Deployed husband having an affair seeking divorce to marry the OW..is it a midlife crisis? HELP

I just need some clarity or someone to tell me what to do. I have been married to my husband for 11 years this past August. He is now 30 and I am 34. We have an 11 yr old daughter. We have had a wonderful marriage, although my husband was unfaithful to me the first year of our marriage when he was 19. We worked through it and have had a strong marriage. My husband joined the US Army 2 years ago and has been deployed to Iraq now for almost 15 months. About4 months ago he announced to me while still in Iraq that he was in love with a soldier there with him and wants a divorce to marry her. She is a 25 year old 2 time divorcee with 2 children to two different dads. I am utterly devestated. We had a wonderful marriage. Even during his deployment things were good. We communicated on a daily basis, had some intimate moments when we could via phone or emails to keep the fires going. I suspected nothing. I have sought counseling here where we are stationed and have been told that it sounds like he is going through a midlife crisis. He tells me that he feels like he needs freedom and that being deployed has given him a taste of that freedom and he likes it. But he intends to marry this girl so he really isnt getting this freedom he speaks of. I just an confused. I want to reach out and help him and bring him back home to our family. He has been telling his parents who tell him that he is making a mistake and throwing away a good thing, that its his mistake to make, but this is what he wants.

Do I just let him go through this and hope it comes out in my favor, that once he returns from Iraq his eyes will be opened? We still speak daily when he calls to speak to our daughter, but the divorce or counseling is subjects he refuses to budge on. He is adament he knows what he is doing and yes he knows what he is losing. I am so alone. I love this man. He is my best friend in the world.