How can I break off an affair?

I am married to a man I love. I'm not sure how it happened but I have been having with another man for 2 and a half years now. We are very discreet and I'm sure my husband doesn't know. My husband has not been interested or able to have for the past 5 years due to cancer. My self-esteem is very very low because of this and the guilt is horrible because I do love my husband. What kind of woman would have an affair when her husband has cancer? I need to end this for my own sanity and I do not know what to do. I want to be there for my husband and do not want to lie or cheat any more.