Brooke Shields Says ‘You’re Never Relieved’ as a Parent and There Are Always ‘New Worries’ (Exclusive)

Brooke Shields is on the brink of becoming an empty nester — but she says her parenting journey is far from over.

The model and actress, 58, tells PEOPLE in this week's issue that she thought she would feel "relieved" when she reached the point where her two daughters — Rowan, 20, and Grier, 18 — were all grown up and living independently. Instead, she's discovered that moms never really stop worrying about their kids.

"You get on vacation and all you do is think about where they are and what they're doing. It's like there's really no relief," she says of the times she and husband Chris Henchy have gone away by themselves. "It's like people say, 'Oh, it's going to be great. Just when they start to be able to walk. You don't have to carry them everywhere. Then they're walking all over and you're worried about them falling off the stairs and in the pool and down the street, and then they're able to walk.' "

Shields continues, "And every step of the way, you think it's going to be a relief. And then a whole new set of worries just smacks you in the face."

Even though her daughters are entering adulthood, the Mother of the Bride star says she still doesn't feel like she can ease up on the worrying and mental load that comes with being a parent.

"What is the expression? Wearing your heart on your sleeve? This is like a hard suit. A whole body suit of a heart," she says, describing the experience of having children.

Come September, when Grier heads off to college, she and Henchy, 60, will find themselves with an empty house — a prospect Shields says she initially looked forward to but now has her feeling off-kilter.

"[I'm] not ready. I thought I would be relieved, but I don't know. I'll see if when I get there, but I think I'm just going to be even... the thought of no longer living 24/7 in the house with these people, that I've raised, it's just a very foreign … it's like going to a totally foreign territory," she tells PEOPLE.

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Shields also says it's been an adjustment to watch her girls become independent, young women, almost as if she is meeting a new version of them.

"I said to my younger daughter, 'You're going to need to give me a little grace period because I have to get to know you. For your whole life since the day you were born, I've told you when to sleep, when to wake up, when to eat, what to eat, what not to eat, what to say, what not to say, how to dress, when not to dress. And I've dictated in a certain sense every single thing about them," she explains.

"And then they start revealing their individual selves to you. And you have to realize, 'Oh, that's a person,' the star continues. "I said to my daughter, I'm like, 'I have to get introduced to you. Because I only know you as doing what I say you should do. And you're not a baby anymore. And I need a little bit of time to re-acclimate to you as the young woman.' "

Shields previously opened up about her close bond with her daughters during an appearance on SiriusXM's The Jess Cagle Show on May 2. She said she and the kids still have sleepovers in her bed whenever Henchy is out of town.

"My younger daughter just turned 18. You know, the minute my husband is on any type of work trip or gone or something, you know, they still sleep in the bed with me," she shared. "We still watch rom-coms. They're my babies and they will never not be."

For more on Brooke Shields, pick up the latest issue of PEOPLE, on newsstands now, or subscribe here.