Three years ago, Bryant Park Hotel doorman Greg Smith, 46, once a diehard supporter of the Clintons, put his life on hold to cam paign for Barack Obama. Shortly after The Post fea tured Smith's conversion on the front page in April 2007, Obama called the doorman and asked him to introduce him on the cam paign trail. Now, 18 months after Obama's inaugura tion, Smith says he's losing hope for the change he once believed in. He tells ANNIE KARNI his story.