QPRD, you could be me. Together 14 years, married 11, two lovely children.
Like halohasslipped, I had an EA. Unlike her (and her life sounds very like mine too) I am going to get out.
I wish I hadn't had the EA. Was it a symptom or a cause of the marriage problems? It's hard to know when you are in it, but in retrospect it was neither. Things weren't right but it was actually rather than a symptom or a cause of this, a distraction that made it easier to ignore the marriage problems. I had my heart comprehensively broken and wish I had never met OM. Only the kids stopped me killing myself, he hurt me really badly, and I was broken by my bad marriage anyway, though OM didn't know this.
All I will say is, listen to the people on here. When you are unhappy, an OM is an intoxicating escape. It will end in tears any which way. OM may break your heart, and your marriage is still screwed, you are back to square one but with a broken heart. Even if OM was a long term prospect, your marriage break up would be seen as due to him and you would be the wicked witch. Don' t put yourself in that position.
OM muddies the waters. Cut him out of the equation and think about what you really want with respect to your marriage.
Trust me, I know how hard it is, and how very very long it takes to make a break, how much you try to avert it, how terrifying it is. I've only just decided, I am going to my parents the day after tomorrow and they are going to help me see a way through, because dh will not do this without fighting tooth and nail.
Feel for you and wish you luck sorting all this out. You are not alone :)