I Went Through the Motions with My Wife, Only to Find It Was Her Best Friend

Help Me Find a Book, Fellow Readers! I Only Remember This One Line From It

Little did she know that time was the most merciless thing of all. When they met again, fifty years had already passed. She had gone from a young girl in her prime to a white-haired old woman, while he looked exactly as he had back then. She knew that immortals and mortals belonged to different worlds. Gazing at that familiar face, she pondered for a long time.