John F. Kennedy's intern admitted to the Daily News yesterday: "I am the Mimi." Marion (Mimi) Fahnestock, now 60, called it a huge weight off her shoulders to finally reveal her affair with the dashing young President four decades ago. "The gift for me is that this allowed me to tell my two married daughters a secret that I've been holding for 41 years," she said. "It's a huge relief." "It's all true," said Fahnestock, sitting in a pew in Manhattan's Fifth Avenue Presbyterian Church, where she works as an administrator. Referring to stories in The News this week detailing...