He left us for another woman

A little over a week ago my husband walked out on me and my two boys. My 7 year old is his step son and is taking things very hard. My youngest has just turned 12 month so he doesn't know thankfully. basically he walked out and wouldn't speak for days then he just came and got his stuff. He started being all nice to me but insisted we had to get a divorce. I've never been so crushed. He just said that he didnt love me. Anyway I thought he was staying with his mother but he was actually staying in a caravan on the site that our new house is getting built. I drove down to pick up some things and went in and a woman was lying down on the sofa. So that's that! I just don't know where to go from here. I'm Just stunned. I knew in the back of my mind another woman had to be part of all this but just had to see it for myself. I'm glad I know what kind of man he is! I just don't know how to move on from here. I'm so lost. I feel for my kids the most.