A Vow to Cherish (1999)

John and Ellen have a picture-perfect life: while John runs a successful business, Ellen works as a schoolteacher, and they've raised two beautiful children. But that picture is shattered when Ellen is diagnosed with a devastating disease. The couple's relationship faces its hardest test yet, and before long, John's business starts to suffer and his eye begins to wander after he finds a lovely new jogging partner.