ABC News' Taylor Behrendt reports:

In a commercial for the diet franchise Weight Watchers, singer Jessica Simpson shows off the slender figure that made her so famous.

It's a far cry from the Simpson who struggled with shedding the weight she'd gained in her two pregnancies. Those struggles were plastered all over the cover of magazines.

"I was so insecure - I couldn't even believe what I weighed," Simpson said in an interview with "Good Morning America" correspondent Bianna Golodryga.

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In the months following the birth of her daughter, Maxwell, in 2012, Simpson lost 50 pounds. Five months later, she was pregnant again with her son, Ace, who is now 7 months old.

"I don't think that I ever expected myself to see the numbers that I have seen on the scale," the 33-year-old fashion mogul said, referring to her weight after she had her second child.

Simpson also said women shouldn't be ashamed of pregnancy weight gain.

"You are creating a life, and honestly it's not easy," she said. "It is a really hard thing to go through. And you deserve to eat your ice cream, you deserve to have your pancakes. Do whatever you want to do and then afterwards just know that you have to work really hard.

"I had to learn that with my first and second pregnancy. My first pregnancy I ate a lot more, but my second pregnancy I ate enough, like I ate what I wanted to eat but I stayed active and I think because I had gained so much weight with my first that I had to stay active throughout my second because I didn't know what to do with myself if I didn't."

Jessica Simpson shared her healthy lifestyle tips on "Good Morning America." (Credit: Kristin Burns)

Staying active and following a healthy diet are important to her.

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"For breakfast I am an oatmeal girl," she said. "Lunch, I do like bread so I will have a sandwich or something. But for dinner I try (to) keep it super clean."

She says she gets her motivation from fiancé Eric Johnson, a former NFL tight end.

She's also busy planning her upcoming wedding. Although she didn't share the date, she said "it will be soon enough."

And in the immediate future, Simpson said she'd like to get back to her roots: singing.

"I want to sing again," she said. "I want my daughter to see me on stage. My fiancé has never even watched me perform. Which is crazy. And - but that is my passion, that is my love, like, that is everything that I am."

Jessica Simpson's post-weight loss body. (Credit: Kristin Burns)

Despite her drive to return to her passion, being a mother is her favorite job of all.

"If I can give my kids anything it is just about support and their dreams and who they want to be," she said.