The Brit spent three years on crutches as she struggled with the condition and now realises that her life back then turned her into an anxious young woman.
In a new interview with The Wall Street Journal, the 34-year-old says, "Feeling stupid is not a nice thing. I wasn't really good at anything. I was relatively good at home economics, at making cakes, and I was quite sporty, but I had juvenile arthritis from the ages of 12 to 15, so I was on crutches."
"It was extremely painful... Anxiety was part of my life at that age, but I didn't realise that was what it was until my mid-20s."
Foy has since found ways to control her anxiety, telling the WSJ a meditation app called Calm has really helped.
"It's not as bad as it was, but that's through a lot of work, doing things that I never thought I would do," Claire adds. "I know that I need to catch myself early in a process of overthinking."
"I don’t feel like I played The Queen. I know this sounds trite, but I feel like I played Elizabeth Mountbatten,” says Claire Foy of her role on The Crown https://t.co/PgwiyfB2U3— The Wall Street Journal (@WSJ) 9 January 2019
And "The Crown" star admits her three-year-old daughter, Ivy Rose, has made it easier to make good life choices.
"The first AD (assistant director) would ask me if we could go an hour over," she recalls of filming the first season of the hit show. "So then I am just like, 'What does everyone want to do? Do we want the overtime or do we all want to go home because we have been working eight days straight?' "
"Time is f**king precious, and making a TV programme is really important, but getting back in time for my daughter's bedtime is far more important to me."