The 36-year-old jazz-pop musician said Winehouse was obsessed with music, reported Femalefirst.
"I thought it would be the music that would bring her back. When she didn't start sounding right, I thought that would be enough to make her stop and get well. I knew how much sounding right meant to her. But it didn't. It's just sad," he said.
Cullum said that in her early days the late singer, who died at the age of 27, never even wanted to go out partying after a concert.
"But this was way before she hit the height of her fame. The Amy I knew was just obsessed with music. She was always a character but music just came first. When we were on tour, she didn't want to go out after a gig, she wanted to go back to the hotel and listen to music. We lost touch when she became famous and then...," he said.