In “How to Break Your Addiction to a Person,” Howard Halpern says that “the most tragic and self-defeating addiction is an addiction to someone who is tied to someone else, particularly by marriage.”
He offers six guidelines to help you determine if you are fooling yourself into harmful addictive behavior, and how to end your addiction.

1. Unless there is hard and firm evidence that he is moving toward an explicit and concrete change in his commitments, stop deluding yourself that it will all work out.
2. You will also have to stop deluding yourself that you are more important to him than his spouse, his marriage, and his children. If you are, he would leave and be with you. Marriage and family can be very powerful investments.
3. You will have to set a reasonable time limit in your mind as to how long you will wait to see if he will make any changes in his status and commitments. And then, if there are no changes, you will have to adhere to that deadline or you can drift on endlessly.
4. Stop idealizing him (or her). Note that the game he may be playing with you–of giving you enough to keep you involved but not the commitment you want–is not a loving game. It means that in this regard he is not Mr. Clean, Mr. Nice Guy, Mr. Ultra Desirable, or Mr. Mature. Rather, there is a childlike selfishness at work in him in regard to both you and his spouse, and you ought to look at that squarely.
5. It can be useful to prepare your friends to be there for you when you go through the inevitable withdrawal symptoms and to help you to maintain your resolve to break it when the going gets rough.
6. In general, and perhaps most important, deal with the probability that if you have been choosing to remain with a person who is committed to someone else, you may be avoiding having a close relationship with a man (or woman) of your own. An exploration of your motives could be quite helpful to you.