"I felt my heart literally open up for the first time and like wrap itself around her. It was profound," Applegate, 39, tells PEOPLE in this week's cover story. "And I'm more in love with her every minute of the day."
Back home in the L.A. house she shares with musician fiancé Martyn LeNoble, 41, Applegate has been relishing motherhood. "I want to be the ultimate burper and diaper-changer, I want to be the ultimate at all of it," she says.
"Sometimes I stand there going, 'I'm not doing any of this right!' And then I get this big man belch out of her and I go, 'Ah, we accomplished this together.' "
It's those little moments that now mean the most to Applegate, who opted for a double mastectomy after being diagnosed with breast cancer in 2008.
"I've had to talk about that word [cancer] more than I've ever had to talk about a word in my life," she says. "After a while you're like, 'There's got to be something else to me.' "
Daughter Sadie has now given Applegate a new word to focus on: Mom.
"Yeah, now I'm Mom. She's healed me in so many ways," says the actress. "She's just made my life so much better. She's opened my whole soul."
For more of Applegate's exclusive interview, including photos of the family, pick up this week's PEOPLE, on newsstands Friday