Hayden Panettiere Says Her Body ‘Ballooned’ Days After Her Brother’s Death and Triggered ​​Agoraphobia (Exclusive)

Hayden Panettiere is opening up about the toll grief took on her body after her younger brother Jansen died unexpectedly in 2023.

In the days after learning her only sibling, also an actor, had died at age 28 of an undiagnosed enlarged heart, she tells PEOPLE that her entire body began swelling up, and she was powerless to stop it.

“I just ballooned out,” Panettiere, 35, says of her rapid 40-pound. weight gain. “It didn’t matter what I did, what I ate. I know stress and cortisol running through your body can do that. Now I think my body was protecting itself, shielding itself from the world.”

She knows her sorrow over losing Jansen was the cause.

“He was my only sibling, and it was my job to protect him,” she says. “When I lost him, I felt like I lost half of my soul…I will always be heartbroken about [him], I will never be able to get over it. No matter how many years go by, I will never get over his loss,” she says.

The weight gain only made the months following worse — Panettiere says she was barely able to leave the house.

“I had to see horrific paparazzi pictures of myself coming out of Jansen’s funeral, which happened in a very private place, and it was shocking,” she says. “I didn’t recognize myself. My agoraphobia came out, which is something I’ve struggled with in the past.”

Panettiere has previously been open about the struggles she went through growing up on-camera —especially in 2006 after she was cast as the superhero cheerleader Claire Bennet in the hit series Heroes at 17.

“Having grown up in this industry, you’re terrified if you don’t look decent when you walk out the door,” she says. “That started happening for me when I was 16, where you’re already at an age where you don’t feel great about your changing body — and you certainly don’t need grown men commenting on [our] insecurities.”

She says that returning to work on Nashville three months after having daughter Kaya didn’t help matters.

“No woman should have to be on camera unless they want to be three months after they give birth,” she says. “I’d just be stuffing every ounce of flub into Spanx, and it added to my depression and what I thought of myself. They say you should speak to yourself the way you would speak to your friends, but I was very, very guilty of speaking to myself horribly.”

Everything for Panettiere changed for the better when she was introduced to celebrity trainer Marnie Alton, who helped the actress take back control of her physical health.

“We’d take these long, beautiful walks where we could vent, and it would be this therapy session,” Panettiere says. “She empowered me.”

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Alton calls Panettiere “so courageous.” Adding, “She was wanting to make a big change in her life, and in order to do that, you have to be really honest about where you’re at in the now.”

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The endorphins Panettiere was getting from burning fat elevated her mood, too. “My body just started reacting, not just from the working out. It allowed me to release the stress, the high expectations I’d always put on myself,” she says.

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As her insecurities faded, so did her agoraphobia. “There’s nothing like looking in the mirror and feeling like you look good enough to walk out the door,” she says.

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These days the star is learning how to balance her grief while getting back into her acting career. On Sept. 27 she’ll celebrate the premiere of her new film Amber Alert.

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“It’s nice being back. I waited for a while to make sure I was making the right choice about what project I was going to do first,” she says. “I got an amazing new team who I love and I could recreate what I wanted to see my career path be.”

To read more about Hayden Panettiere’s life now, pick up the latest issue of PEOPLE, on stands Friday.