Actually, I have many mistakes in life. However, I think you're looking for one single defining moment that we feel we made a decision that changed our lives negatively forever. That decision, for me, was the decision to make a phone call at the precise moment I did. It was a phone call I had made to my husband who had left for a friend of mine. He had come back, a year later, to tell me he made a mistake. I said we'd have to go to counseling. That phone call is my greatest regret. To cope with my grief over that decision, I re-wrote the scenario with a GOOD outcome in a short story that I call "Crack in the Sidewalk." (I have several versions of this and the one on hubpages still needs editing- the ending needs expanding - that's my writer's embarrassment speaking here.) ANYWAY, In the story, I go over to the other woman's house and confront her telling her what our marriage means to me and being quite strong in fighting for my marriage. In real life, when I made the phone call and heard my husband say, "Can I call you back later?" I knew she was at his apt. After a series of soap-opera events that fortunately did not turn tragic, I told the other woman "Fine, you can have him." and was done. I really didn't mean to leave her with that impression, nor my husband. I wanted my family and didn't fight for it hard enough. If I hadn't made that one phone call, we would have continued with counseling and I think we could have made it. As it turned out, many lives were terribly emotionally destroyed - especially the children. My point is, don't try to save face when you happen to be in a situation like this by denying how much you want your marriage. People will tell you, "Move on with your life," "He'll do it again," and on and on. They're not comfortable with ambiguity. But rational, honest dealing with the situation is what's called for and if you need help going about it with reason and dignity, get the help. Don't let people convince you of something you know in your heart is not right for you. Don't play games or try to save face. Too much is riding on your decision. "Measure seven times, cut once."