How do I make him forget about me?

About 12 years ago, I met a man with whom we connected instantly and later became best friends. He was married and I had a boyfriend.

Since I did not have any romantic feelings for my new best friend, I did not see how this would present a problem in our friendship. With time, he started confiding in me about the problems in his marriage. I was not married, but I always gave him what I considered the best advice.

Two years ago, during one of those sessions when he was pouring his heart out, he mentioned that he wished he had met me before getting married. He said he would have chosen to marry me and not his wife.

He then suggested either divorcing his wife, having a secret affair with me or becoming his second wife. I do not condone divorce or adultery, so I ended our friendship immediately so he could go and resolve the challenges in his marriage.

He went quiet until six months ago when he started calling me incessantly. He says he would rather die than continue living without me. I do not want to break their marriage up.

Besides, I'm engaged and we're getting married in August. How can I make my friend forget about me and focus on his wife?

Name withheld