You're Having An Affair: When Do You Tell Your Spouse?

You're married but you've become emotionally attached and physically intimate to someone else outside your marriage.  You're struggling with guilt - after all, you don't consider yourself a bad person - but it has happened.  Are you happier with the other person than you are with your spouse? Does this mean the end of your marriage? Are you ready to confess to your spouse and prepare for a divorce? Or is this just a temporary fling you hope your spouse never finds out about? Is it something you don't intend to end? If not, then when do you tell your spouse that you're in love with someone else? And how do you tell them in the least painful way?