I am so sorry you are going through this. Sadly, many of understand what you are feeling because we have been there too. It makes me SO mad too. You know if that girl was not interested, your husband would be home trying to work it out. I don't know how women can do this to other women. But one day he will do the same to her and she will be the one crying.
Be strong. Hold onto your family, your friends, your faith, and to the women in this room. Scream, cry, curse, beat the walls if you have to. But that feeling of total devastation, feeling like you can't carry on another second, it will begin ease and soon you will raise your head and notice that life is still worth living, and it may be even better than it was with your husband. In time you will wonder why you hung on to your marriage for so long. Your happiness is on the horizon you just have to wait for the sun to come up again.