Simpson can be seen sporting a noticeably slimmer figure in the latest commercials for the diet franchise, Weight Watchers.
Her struggle with her weight was plastered all over celebrity magazines while the singer was trying to shed extra baby pounds.
"I was so insecure...I couldn't believe what I weighed," she said in an interview with "Good Morning America" correspondent Bianna Golodryga.
After giving birth to her daughter, Maxwell, in 2012, Simpson shed 50 pounds. Five months later, she was pregnant again with her son, Ace, who is now 7 months old.
She said she never expected to see the scale climb so high after she had her second child.
While she believes a woman should be able to indulge while pregnant, Simpson said she learned from her first pregnancy that after birth, you really have to work hard to get back to your pre-pregnancy weight.
"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," she said.
Simpson said she tries to stay active and stick to a healthy diet. For example, she sticks with oatmeal for breakfast, has a sandwich for lunch (because she says she likes her bread) and for dinner, she tries to keep things clean and light.
She says she gets her motivation from fiancé Eric Johnson, a former NFL tight end.
Simpson is also busy planning her upcoming wedding. Although she didn't share the date, she said "it will be soon enough."
The fashion mogul said she really wants to get back to her roots, which is 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," she said. "That is my passion, that is my love, like, that is everything that I am."
Even with that said, she said being a mom is her favorite job of all.
ABC News contributed to this report.