The 24-year-old pop star hilariously claims that her new, healthier lifestyle stems from a desire to "live forever," but admits that keeping off the pounds has been challenging.
"I did fluctuate and learned that the quick way, 'cause I never ever lost that much weight before," she tells ET of her recent 20-pound weight loss. "I immediately noticed, like, 'Oh, there's a ten back' and then I had to lose it again. So that's a lesson to learn."
But for Trainor, it's about more than just what the scale says.
"I just want to live forever, man. I wanna stay looking young forever," she quips. "I stopped drinking, stopped doing anything bad to my face. I don't want to deal with acne anymore -- I'm 24, I'm over it, and I just want to live 'til I'm 106."
"He really changed my life. He showed me how working out can be fun. I didn't believe it, but now it's true and it feels so good when you're sore -- I love that feeling now, I'm addicted to it," she says. "He cooks for me and taught me to cook. I never knew how to cook. He taught me, like, secrets how to make your food taste great but also be healthy for you."