Kevin Federline claims his and Britney Spears ' son Sean Preston was "terrified" to stay with his mom during some of their most turbulent years.

In his memoir, " You Thought You Knew ," Federline recalls gut-wrenching nights comforting the boy, who he says would cry and cling to him whenever it was time to visit Britney. Listenin

"Every time Preston had to go to his mom's, they had to pry him off my neck," Kevin reads in the audiobook version. "He'd cry, scream, and hold on with everything he had."

Kevin says the anxiety bled into nights at home with Preston ... describing how Preston couldn't sleep unless Kevin stayed by his side. Gallery Launch Gallery

"Even the slightest movement would wake him in a panic," Kevin writes. "I'd sit beside his bed, my finger resti

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