"We are a people who do not want to keep much of the past in our heads," Lillian Hellman once wrote. "It is considered unhealthy in America to remember mistakes, neurotic to think about them, psychotic to dwell upon them." Yet who in their lifetime has never regretted a lost love, a missed opportunity, a path not taken? Indeed, regret is perhaps a universal experience, but while poets and novelists have long explored its complexities, very little...