Antonia, here's two very good responses you can use when blamed by your husband for exposing him:
1. I decided I am no longer willing to lie to cover up your affair.
2. Everything I've done, I've done to try to fight for our marriage and our family.
Is OW married? Did you expose to her husband? If so, another great response is:
3. I felt that she had a right to know, and to make her own decisions for her family. It wasn't fair that she was the only one of the four immediately affected who didn't know the truth.
