Alison Sweeney Poses in a Bikini
04/05/2011 AT 01:40 PM EDT
"I can't even tell you what my life would be without the show," Sweeney, 34, says. "I'm a more confident person and certainly more well informed, and that confidence I have comes from knowing I'm doing what I need to do and working hard for it."
To get bikini-ready, Sweeney, who wrote The Mommy Diet, embarked upon what she calls "shape-up week." Several times a year, she says, "I cut the crap out of my diet – cheese, dairy, carbs like bread and pasta and fatty meats." Sweeney, a popcorn fanatic, instead snacks on sliced bell peppers and cucumbers, and she exercises every day.
That's what helped her drop from a size 4 to a size 2, just in time for the photo shoot.
Since joining The Biggest Loser in 2007, Sweeney has learned to control meal and snack portions, exercise regularly (she goes to group spinning and yoga classes near her L.A. home) and indulge only in moderation.
"I'll have wine or a piece of cake once in a while," says Sweeney. "But I don't look at it as sliding backwards, even if I go a whole week without working out. I don't dwell on it and beat myself up – I just try to have a healthier day tomorrow."
The Biggest Loser airs Tuesday's (8 p.m. ET) on NBC.
To read more about Sweeney's transformation, pick up the latest issue of PEOPLE, on newsstands now