REVEALED: WHY O.J.'S AGENT FINALLY BETRAYED HIM

A boyhood fascination with O.J. Simpson turned into a nightmare for his former sports agent - who's written a tell-all book revealing Simpson confessed to killing his ex-wife, an insider tells The ENQUIRER.

"I just got tired of covering up for O.J. all these years," Mike Gilbert writes in his bombshell new book: "How I Helped O.J. Get Away with Murder: The Shocking Inside Story of Violence, Loyalty, Regret, and Remorse."

"He's a liar, he's a killer, and he's a batterer, and he's never admitted any responsibility for any of the bad things he's done."

Gilbert, who lives in central California, idolized Heisman Trophy winner Simpson as a child. A sports agent in the 1970s and '80s, he was introduced to O.J. by another famous football player, Marcus Allen. By the time of the murders, Gilbert was in Simpson's innermost circle.

Gilbert admits to helping Simpson earn more than $2 million by brokering autographs O.J. signed from his Los Angeles County Jail cell after he was charged with murdering his ex-wife Nicole and waiter Ron Goldman in June 1994.

And in his book, Gilbert describes the fateful night when Simpson confessed to the double murder.

Pick up The ENQUIRER for all the details!