The 62-year-old star has tried to help other people overcome their problems since she launched her talk show in 1986 and admits she has always felt her own struggle didn't fit with her public image.
Her best friend Gayle King said: "This food struggle is something that she's been battling since, I think, the public met Oprah.
"I have suffered publicly -- fasting, eating no carbs, green juices, protein shakes, blah blah blah.
"My public struggle with weight has always embarrassed me because it felt off principle."
"How can I give everybody else everything they need, and I still can't give this to myself?"
But thanks to her plan with Weight Watchers, Oprah is finally happy with her weight, though she thinks sticking to the points-based regime will be something she does for the rest of her life.
She told the new issue of People magazine: "I will probably be counting points for the rest of my life.
"Life itself feels sharper. I have more energy to give to the things that really matter."
Oprah revealed in a Weight Watchers commercial last month that she has shed 40lbs thanks to the plan.
She said: "Since I've started Weight Watchers, I've lost over 40 lbs. I can honestly tell you, I struggle no more.
"I'm eating everything I love - tacos, pasta. I've never felt deprived."
Oprah joined Weight Watchers in August 2015 and by January last year had managed to shed as much as 26 lbs. through healthy eating.
And in May 2016, the TV host decided to increase her dedication to the cause, vowing to record at least 10,000 steps a day every day in a bid to lose even more weight.
She said: "I try to do something every day that allows me to feel active, and I don't make myself crazy about it.
"I just know that movement and flexibility, particularly the older you get, is what makes you feel alive. So I don't want to just be alive, I want to feel it."
Earlier this year, Oprah explained how her weight loss has changed her relationship with Stedman Graham, her long-time partner.
Oprah - who is a shareholder in Weight Watchers - said: "I would like him to pick me up and carry me to the pool.
"I've lost enough weight, he can pick me up and carry me to the pool. I can straddle him without breaking his back."