Spears, 29, told Menounos that even though Maddie, who was 8 at the time, was riding the vehicle "with every safety measure that could be taken, she just somehow or another, drove into the water, and we dove in and we were unable to rescue her.”
The younger sister of Britney Spears said she was convinced Maddie was going to die after the crash.
“When we were finally able to get her out of the water ... we thought she was gone," she recalled. "We thought we’d lost our daughter. That moment, I felt everything that you can feel, I think, as far as the worst. This is the worst.
"There's nothing worse than looking at your child and just feeling that you've failed her. And I didn't want her to think that I couldn't save her, that I didn’t try to save her. That was my biggest worry," she said.
Maddie was airlifted to a hospital after a firefighter told Spears they found a pulse. Despite that, Spears recalled that hope was grim.
“It wasn’t looking good. She wasn't responding to anything," Spears said.
The singer said she asked a priest to come in to read last rites, but then things took a turn for the better.
“He went to put the oil on her and read the rites and she sat up and started kicking, and her hands and started grabbing at all the things," she said.
"That was our first sign that she was there,” she added, noting even the priest was stunned by what he saw.
Spears said the incident was a defining moment for her.
"I’ve faced my worst fear now," she said. "What else can I mess up or do wrong that’s gonna be as horrible as that? Nothing. There's nothing. God gave me the blessing of giving me my daughter back. I lost her and I got her back. So I don't get to make any excuses. I've been given the biggest blessing you can be given.
“That’s the worst moment I’ve ever had ... but I was given her back and for so many, that’s not the case. So I'm not allowed to waste a day on this Earth complaining or being ungrateful."
Spears, who said her now 11-year-old daughter has completely recovered, was 17 when she had Maddie with ex-fiancé Casey Aldridge. She also has another daughter, Ivy, 2, with husband Jamie Watson.
“I’m so lucky for my little girls,” she said.