MARIAH Carey is said to be scrambling to complete her new album, “Memoirs of an Imperfect Angel.” “She’s getting very nervous now that Whitney Houston is coming back,” said our source. We’re told Carey signed with manager Chris Lighty four months ago, but the album still isn’t finished. Lighty told Page Six by e-mail, “My management company is dealing with [Mariah’s] music, brand extensions and commercial opportunities. We love Mariah.” Carey hasn’t had a music manager since splitting with Benny Medina last September and has only released one single — “Obsessed” — off the album so far. We tried to get more details from Lighty, but he told us he was “at the beach” and couldn’t talk. When we called her p.r. firm, a rep said, “We are no longer working with Mariah.”