Fit and fab at 40! Jessica Simpson’s dealt with her fair share of weight woes over the years, but the blonde beauty proved she’s never looked or felt better when she slipped into a pair of 14-year-old True Religion jeans to mark her birthday.

“I figured that since I’m in the final hours of my 30s, I’d give them another try.” she said on July 9.

“Hello 40, so nice to meet you.”

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The former pop star got candid about her fluctuating weight in her memoir, Open Book, revealing that her struggles began when she entered showbiz at age 17.

Since then, she’s experienced countless highs and lows, from her head-turning physique in 2005’s Dukes of Hazzard — which she says “created a gold standard” and unrealistic expectations about her body — to admitting she took diet pills, restricted her food intake and eventually underwent two tummy tucks.

“It’s taken a long time for Jessica to feel comfortable in her own skin,” a source said. “But now she’s telling friends she thinks she looks better than ever!”

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It was just last year that the star tipped the scales at 240 pounds, which spurred her to turn to trusted celeb trainer Harley Pasternak, who helped her drop a whopping 100 pounds.

“Jess works out three times a week, but her routine’s very simple: a brisk 12,000-step walk followed by light weights, squats and lunges,” explained the source. “She even does pliés while she’s cooking.”

The fashion designer also revamped her diet. She now sticks to three meals a day.

“She keeps it simple, like a smoothie for breakfast and a salad with grilled salmon for lunch,” followed by a nutritious dinner, the source noted.

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What’s more, if she’s in the mood for dessert, she’ll have half of an oatmeal raisin cookie. Simpson’s also learned to cook “healthy.”

“If she wants Italian, she’ll make roasted eggplant casserole with spinach, goat cheese and tomato sauce from scratch. That way she can control the fat and sodium content.”

But the “Irresistible” singer does get to eat one cheat meal per week!

“She and her husband (Eric Johnson) have a date right where they order from their favorite restaurant, and Jess gets whatever her heart desires without feeling guilty“ the source shared to OK!.

“The next day, it’s back to the program.”

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