I am amazed to see how many people have written to say that after reading the book, they decided their relationship was too good to leave. Too good to leave deserves your full commitment. The major fear is being alone and never finding love again. The way to deal with it is to make certain you stay connected to people and things you care about. As long as you care about people and things that matter to you in your life, you will be happier by definition than staying in a relationship that was too bad to stay in.
Help is available for people with doubts.
I am 51 have a very good sex life. Alaska, expect more of the same. I thought affairs were fatal, but that is not the case. It is not the affair, but the things around it. Being unable to forgive or having more love to give or feeling he is a liar.