On his 80th birthday, Richard M. Nixon confided that it was a day he had been dreading, except when he considered the alternative. "The secret to a long life," he said, is "never look back, look forward. You've got to have something to live for, otherwise you cease to live." The occasion was celebrated at the Richard M. Nixon Presidential Library & Birthplace in Yorba Linda as the library's volunteers and their grandchildren led the public in the singing of "Happy Birthday" as the former President listened via telephone hook-up from his home in Woodcliff Lake, N.J. He said he and his wife, Pat, spent his birthday, as they do all major occasions, with family.