The actress says she was dubbed "blubber" by her classmates.
"Other girls teased me terribly," Winslet, who stars in the new film "Little Children," says in the new issue of Parade magazine. "I was bullied. I'd just put my head down and get on with it. That was my means of survival."
Later, at age 15, a boy named Stephen Tredre would enter her life and become her first love. "Stephen made me feel secure and embraced," she says. "He was the most important person in my life, next to my family."
In 1997, Tredre died of bone cancer during the opening week of "Titanic." Winslet missed the film's L.A. premiere to attend his funeral. "Looking back, I see what I was dealing with when Titanic came out. I had a lot of pain, and I was confused about who I was," she says.
The actress details her failed marriage with assistant director James Threapleton; they met on the set of her film "Hideous Kinky" and split in 2001. They have one child, Mia.
"I thought I wanted to be with him," Winslet says. "I was dealing with the pain of having lost Stephen and 'Titanic' coming out. Jim was just a regular guy, and that had a big impact on me."
After that marriage ended she met director Sam Mendes, to whom she's been married since 2003.
Upon their first meeting, "I walked away dazzled, thinking, 'What the hell was that?' I knew this was the person I was meant to be with. I was terrified. I didn't tell anyone I had this feeling, not even my mum. I had no idea if Sam felt anything for me."
After they began seriously dating, the British press accused Winslet of "trading up" to a "better class of man."
"It was shattering," she says of the media hubbub surrounding her divorce from Threapleton and marriage to Mendes, "but it was nothing compared to losing Stephen."
Winslet and Mendes welcomed their first child, Joe, in 2003.
Still, through it all, Winslet says she believes in fate. "I know it sounds corny, but it was like Sam and I were from the same tribe," she says. "We were meant to meet: Both of us from Reading, both born in the same tiny hospital, Dellwood. Then suddenly, years later, this totally gorgeous, sexy, talented man is in my life? That's fate."
The new issue of Parade magazine is available on Sunday.