From
What Are Plan A and Plan B?"In general, a betrayed spouse's effort to encourage the wayward spouse to end the affair should address all the root causes of the affair, and offer a solid plan for marital recovery. It should not be one-sided, however. The plan should make the wayward spouse and the betrayed spouse equally responsible for following the overall plan.
But plan A, an effort to end the affair with thoughtfulness and care, doesn't always work. In many cases a wayward spouse is so trapped by the addiction that he or she does not have the will-power to do the right thing. Once in a while the fog lifts and the cruelty and tragedy of the affair hits the wayward spouse right between the eyes. In a moment of grief and guilt, he or she promises to end it. But then the pain of withdrawal symptoms often brings back the fog with all its excuses and rationalization, and the affair is on again.
Sometimes a wayward spouse settles into a routine of having his or her cake and eating it too. In an effort to win the wayward spouse back, the betrayed spouse meets emotional needs that the lover cannot meet, while the lover meets emotional needs that the betrayed spouse has not learned to meet. While this competition is excruciatingly painful to the betrayed spouse, and the lover as well, the wayward spouse basks in the warmth of being loved and cared for by two people, with no real motivation to choose one over the other.
So, to avoid an indefinite period of suffering while a wayward spouse vacillates between spouse and lover, and to avoid rewarding the selfish behavior of having needs met by both spouse and lover, if plan A does not work within a reasonable period of time, I recommend plan B."