1. You may think you’re truly in love with your illicit partner, but if your affair should evolve into real relationship, you may experience the same relationship problems [that you have in your own marriage] all over again. (That's what keeps me real, BTW. My xAP is going through his own divorce, has 3 teenaged children, is in a complicated co-parenting arrangement [where the spouses swap out their shared apartment every other week] and lives 500 miles away - lots of fodder there to create unhappiness for me.)
2. Confide in a trusted, non-judgemental friend. Talking to this person can be very healing, and offers an opportunity to reflect back the truth, particularly at a time when the headiness of your affair can cloud your ability to think clearly and honestly. It’s simply a good way to obtain a second opinion on how to end an affair while providing the emotional support that you absolutely need at a time like this.
3. Spending time with a qualified, experienced therapist is a wise step. Not only can this person help save your marriage, they can help you work on areas of your *OWN* life that make you sad, and that possibly created the fertile ground for having your affair to begin with. I began with a therapist who was not right for me, but I didn't give up. I asked people I trust for recommendations, which led me to an outstanding professional who is helping my husband and I get back on track.
4. Finally, and this is a personal decision, think twice about confessing your affair to your spouse. I have chosen not to, since I know it would hurt him deeply, sour our mutual friendships, hurt my own family (who I have not told - but he might be tempeted) and quite likely lead to the end of my marriage. It would also hurt me deeply, and likely throw me into a deep depression.