I don't understand, someone help me please....?

~ My husband and I are getting seperated, while I am pregnant with our second child. He is in the military and has a ton of single friends and wants to go out partying constantly. My daughter and I moved from Montana (where he is stationed) to Illinois so I could be around my family for my pregnancy since he was no where around while we were there living with him. I gave him 6 years of my life and devotion, and just found out he has had a girlfriend basically once I left. My question really is, how is it so easy for him to move on and so quickly? I remain in love with him but never would I go back to him. I just don't get how I can give my time with him and it seems like it doesn't matter what we've been through, because he is with someone else now. Whats worse is we are still married and he been with this girl...