The 34-year-old BAFTA nominee has revealed she grew anxious from a very young age because she had to use crutches to get around due to “extremely painful” inflammation of the joints – and it made her feel “stupid”.
In an interview WSJ. Magazine, she said: “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 realize that was what it was until my mid-20s.”
The ‘First Man’ star has turned to the meditation app Calm to help her manage her “overthinking”.
She said: “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.
“I know that I need to catch myself early in a process of overthinking.”
Claire also opened up about the gruelling filming schedule for Netflix period drama ‘The Crown’ – in which she portrayed Queen Elizabeth II – and how the cast were often asked to stay on longer than planned.
However, being back home for her three-year-old daughter Ivy’s bedtime was “far more important” to her.
The actress – who has her little girl with estranged husband Stephen Campbell Moore – said: “The first AD (assistant director) would ask me if we could go an hour over.
“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.”
The ‘Upstairs, Downstairs’ star previously admitted her anxiety has worsened since becoming an actress and finding fame.
She said: “When you have anxiety, you have anxiety about – I don’t know – crossing the road.
“The thing about it is, it’s not related to anything that would seem logical.
“It’s purely about that feeling in the pit of your stomach, and the feeling that you can’t, because you’re ‘this’ or you’re ‘that’.
“It’s my mind working at a thousand beats a second, and running away with a thought.”
Claire suffered a “breakdown” in her early 20s when she couldn’t cope with her own mental health.
She said: “When I was 23, I had a breakdown. I didn’t sleep or eat for a month. It was awful. My life had changed. I felt overwhelmed.
“I was on stage at the time and working, and I’d just got a really big job (the lead in the BBC’s ‘Little Dorrit’). I just couldn’t cope.”