How can you stop having feelings for a married man?

You have to make a conscious decision to not pursue him, as it is best for all parties involved. Even if his marriage isn't going well, the marriage has no chance with a third party butting in. If it were your husband, you'd want the space to repair the relationship.

Next, it will help if you can identify what it is about the man that you find appealing. Many women become interested in a married man for the way he treats his wife; you want her happiness and/or security. Perhaps, you like the bond you have as he confides in you. Sometimes men need to talk to someone about things that other men aren't interested in hearing about and are too touchy to discuss with the wife. Understand that bond would be with someone else if you were the significant other.

Whatever your reason for your attraction, identify it and look for similar qualities in available men.