Is He Really Happy

My exN of fifteen years is back with a woman he cheated on me with, is expecting a baby but is still married to me. I decided to leave him almost two years ago because of his abusive behavior with our kids and I. I gave so much in my marriage and could not get him to treat us right but now he is with this woman who the kids say he treats kindly and he does not mistreat my children anymore. How could that happen?