"I had all the classic symptoms: extreme joint swelling, different pain, the inability to do certain things that everyone else could," she tells PEOPLE of first being diagnoses with juvenile RA as a child. "That's when I knew that something wasn't right."
Now, the 26-year-old is raising awareness of the disease with the educational campaign Joint Decisions, sharing her story to support others who suffer from the autoimmune disorder. "It's empowering people who live with RA to share in the healthcare decisions that are going to impact their life and their overall wellbeing," she explains.
The biggest misconception she encounters? That only older people have RA.
"Rheumatoid arthritis can affect anybody at any age. Usually when I tell people, they're like, 'Oh right, I think my grandma has that.' They only really associate it with the elderly," Park says. "There's a lot of people like me with arthritis."
Rheumatoid arthritis is a chronic inflammatory condition that causes painful swelling in the lining of the joints and can damage them, according to the Mayo Clinic.
The exact cause is unknown and there is no cure – and about 1.5 million Americans are living with it, according to the Centers for Disease Control and Prevention. Although there is no proof that RA is genetic, Park's mother was diagnosed with the disease five years ago, which Park calls "a role reversal" that has brought them even closer.
Park has tried all sort of treatments over the years, from medications to monitoring her exercise, making sure she doesn't do any strenuous workouts that would hurt her knees and other joins. The challenge is that RA "ebbs and flows," she says.
"Sometimes you need to do a lot, and other times it's very easy and very minimal management," she says. "It's a disease that stays with you for life, so you sort of have to learn how to manage it on a daily basis, and that can mean various degrees of how bad it is."
Courtesy Megan Park
Since Secret Life went off the air in 2013, Park's kept up a steady string of acting jobs, from the romcom What If with Daniel Radcliffe to ABC's The Neighbors. Her latest movie, Room, is an adaptation of Emma Donaghue's dark novel told from the perspective of a child born out of a kidnapping.
She considers herself lucky that living with RA hasn't hurt her career.
"I've been really fortunate that it's never hindered my ability to work," she says. "When I'm at work, I'm hunkering down and making sure I get lots of rest. I'm making that sure that I'm not doing anything in my off time that's going to aggravate my joints.
"But I actually think in a lot of ways, it's helped me understand that everybody has plights, and it's made me more empathetic, which I think has helped me as an artist, when I'm acting. I think it's opened my eyes to, everybody has a story, essentially. You may not know about it, but everybody has something."
Park is also grateful for the support of fiancé Tyler Hilton, an actor and singer she met on the set of Charlie Bartlett in 2006.
Jason Merritt / Getty
Hilton is "a protector," she adds, "and wants to take care of me all the time, but sometimes just the best thing for me is to rest and do what my body's telling me to do. ... Now I feel like he really understands what it is and has a lot, a lot of compassion for it."
The couple will tie the knot with a "low-key" wedding in October ("I have so much to do still!" she says, laughing), and Park hopes the celebration will be a mini Secret Life cast reunion.
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She's still close with everyone from the show, especially Shailene Woodley.
"Shai, since day one, has been my best friend," she says. "Of course I don't get to see her as much as I did then because she's traveling the world all the time, but we're texting almost every day, if not talking. She's wonderful. I'm so proud of her and everything she's accomplished. She's just an amazing soul."