January 21, 2016 10:20 AM

Starring in HBO’s hit series, The Sopranos, when she was 20 years old, actress Jamie-Lynn Sigler was “on top of the world.”

Though she had been diagnosed with Lyme disease the year before, the actress had taken antibiotics “and things had started to go away,” she says. “Then I started to feel this heaviness and tingling in my legs. I remember we were in my apartment in New York and the doctor called my dad and he said, ‘Your daughter has MS. But there’s no reason to believe that she can’t live a perfectly happy, healthy life,” she recalls.

“It was a shock. I didn’t feel sick. My ideas of MS were limited. I thought it meant, ‘Wheelchair.’ I thought it meant your life was over.”

RELATED VIDEO: Newlywed Jamie Lynn Sigler Reveals She Has Been Battling MS for 15 Years

Sigler, 34, was symptom-free “for quite some time” and even starred on Broadway for months as Belle in Beauty and the Beast in 2002. But after her marriage to A.J. Discala ended in 2005, “that’s when things started to kind of go downhill for me,” she says. “I started having a lot of weakness in my right side, but I could still cover it up. And I started having things that are kind of awful and embarrassing like incontinence issues. I holed up for a while.”

Celine Dion on the cover of PEOPLE

For more of Sigler’s revealing interview, including exclusive photographs from her wedding album, pick up this week’s issue of PEOPLE on newsstands Friday

The actress found support through her family, close friends and a few of her Sopranos cast mates, including James Gandolfini who was “really awesome about it,” she says.

“Throughout this whole journey, I’ve been so lucky to have such fiercely protective family and friends because I wouldn’t have been able to do anything without them,” Sigler adds. “This is a disease that can cause depression. When you wake up, you feel like you want to be in bed all day. That’s still how I feel every day. But you fight it.”

For more on living with MS and to contribute to the fight against the disease visit The National Multiple Sclerosis Society.

Now married to pro baseball player Cutter Dykstra, 26, and mom to their two-year-old son, Beau, Sigler is confident about her decision to go public with her illness.

“I’m never going to know why I have this until I live my truth. But it doesn’t define me,” she says. “I know that there are a lot of other people out there struggling. If I can finally be okay and open and honest that I have this, maybe I can inspire other people to do it, too.”

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