Find out why you got involved with a married man to begin with. Some women have underlying psychological issues that make them seek out married men. Once you figure out why it happened, you can either see a counselor or decide on your own how to stop being the other woman.
Tell yourself that you really deserve better. Go over all of the reasons that you are too good to accept the role of the other woman.
Realize that he is not your soul mate, even if your heart says that he is. How can he be the one intended for you when some other woman has already married him? If you break it off and he becomes single at some point, then you can truly decide if he is the one for you.
Think about what people would think, like your mother or father. Would it be a proud moment for your parents to find out that you are with a married man? How about your grandparents? There really is a reason that you don't go around talking about your wonderful man-because he really isn't "yours" at all.
Find solace in spending time with friends. Filling your time with your friends will put your mind on something besides being with him. In fact, getting your mind off of him will be a big step in bringing an end to this affair.
Go out on dates. There are many available men that would love to go out with you. Possibly, you have already been asked out and declined because of your current involvement.