In an interview with The Sun, the recovering addict said: "I was the one who brought drugs into the relationship."
The 30-year-old - who was serving a sentence in a Leeds prison when Winehouse died at her home in Camden, North London last summer - also confessed that he was glad when blood tests showed she had not overdosed.
"I don't know whether it absolves my guilt or responsibility. But it meant I didn't kill my ex-wife," he said.
"When the toxicology results came back I was relieved there were no drugs in her system."
Winehouse was 27 when she was found dead after a suspected drinking binge last July. Fielder-Civil insisted that it was the singer's dependence on alcohol - and not her drug taking - that was her true weakness.
"It seems disrespectful to Amy to assume she was so impressionable she would take [drugs] without making the decision to do so herself," he explained.
"She was actually very strong and independent."
Winehouse and Fielder-Civil married in Florida on May 18, 2007. They remained together until August 2009, when they divorced amid claims of adultery.
"I thought my world had ended," he added, recalling the moment he heard the news of her passing. "We were two peas in a pod."
Fielder-Civil was unable to attend Winehouse's funeral and said he instead held a ceremony for her in his cell. After completing 12 months of jail time for committing acts of burglary and possessing a firearm, he overdosed after reading texts that the star sent him while he was incarcerated.
The former production assistant was hospitalised and had to be placed in a medically-induced coma for two weeks. He claimed that Winehouse came to him in visions to help him pull through.
Fielder-Civil has a son, Jack, with new love Sarah Aspin and the couple recently revealed that they are expecting a second child.
"Amy [just] wanted me to live life with my boy," he said.