Him finding someone else so quickly is another symptom of the disease. It's just a way to numb the pain of being alone. She has nothing to do with you and, believe me I know, that is the hardest thing to let go of. I used to imagine my AH in these happy loving evenings with the woman he was cheating with. He once told me "I'm just with her because I miss you so much" " I pretend she's you all the time, I just don't want to be alone" I pictured candlelight and soft music when actually they were watching cops and drinking Jack Daniels. I deserved better and so do you. It will take time but you will find your way out of this.