What a mess! Please help me?

My husband has threatened divorce many times throughout our marriage, usually just empty threats. Over the past month or so, our marriage has been terrible with him running away, not coming back, moving into the guest room, etc...Then things were going better. They were starting to get OK. Then, he came home and told me he'd already filed for divorce, and he just doesn't want this anymore (we've only been married 2 years and we have a baby). He is completely unwilling to go to counseling or work things out. I found messages and discovered that he's been seeing a woman behind my back. The messages were flirty. I confronted him and he told me she was just a friend and nothing more. A later message I read confirmed that she was just a friend, but that she seems to be excited about his marriage ending and she's poised to just swoop right in there when I'm out of the picture. I can't believe he would suddenly end our marriage because of this person! Anyway, tonight I told him that I wasn't meeting his needs before and that I was so completely sorry for not listening to him when he talked about how important physical intimacy was. I told him I realized it was probably too late, but I needed to say I was sorry. He told me he was really angry that I hadn't understood that until now. We talked a bit more, and he actually seemed interested in hearing what I had to say (unusual lately, as he usually just wants to run away when I'm talking). Then, he abruptly decided he was going to the bar down the street. I asked why he was mad. He said he just needed to be alone with his thoughts. When he came home, he said hello very quickly, rudely said he was tired and tromped upstairs. I asked if he was ok, he said fine, tired. Asked why he was mad (yeah, I know I sound like a huge nag now) he said he's not. He was just being so completely nasty. I said I was sorry and that I thought I was being nice before. He said I was nice. But he was still being really nasty. I thought the fact that he was listening to what I was saying so intently and took the time to go think about it gave a little bit of hope to the situation. It seems it didn't. Is this broken beyond all repair? He's not having an affair, but it seems like he's barreling towards it. I just don't know what to do. I haven't gotten divorce papers yet. I'm so upset. I just want to save my marriage!