First, breathe. If you have children, there is a lot to gain and a lot to lose in this situation so act now as if you're in slow motion. Nothing has to be decided today. If your spouse is forty or close to it, you are probably facing something outside of yourself - outside the two of you. Generally speaking, if you and your husband or wife have loved each other for a long, long time, this is not about you - it may not even be about him/her or the other person. You have to be your MOST HEROIC SELF now; you have to be braver than anyone should ever have to be, again, especially if you have children. Take things slowly - measure 7 times, cut once. Read inspirational quotes or do whatever is healthy to ease the angst. I'm not saying that the other advise in the answers to this question aren't valid and maybe even better advise. No one knows. I'm just saying, take everything into consideration. Right now DO NOT tell the spouse of the other person, DO NOT tell your children, DO NOT tell anyone else in the family. DO start reading about good relationships - why people stay together. Start doing those little things you did for your spouse when you were first married. It will be difficult - you'll want to throw up. Do it anyway - at least for now. I wish that my actions during that time in my life would have been different. I would have done EVERYTHING differently than I did. I lost all that was dear to me. Like anyone who has experienced grief to the bottom of his or her soul, the only way my own experience can at least have some meaning would be if it prevented a tragedy like mine occuring to someone else. But for you, right now, just breathe!