Claire Foy suffered a breakdown in her mid-twenties.
The 37-year-old actress has revealed she turned to professional help after sharing her mental health struggles with her sister.
Claire - who played Queen Elizabeth in 'The Crown' - explained: "I basically was one of those people who was very much - I think it's a lot to do with my upbringing - like I don't need help.
"I can sort it out myself. It's not that big of a problem.
"In my mid-20s, I sort of had a breakdown, and I remember my sister saying, 'I can't help you. You need to see someone professional'.
"That was the first time that I sort of went, 'the problem is bigger than me being able to figure it out.'"
Claire admits that talking about the issue with her friends has helped to transform her life.
She told the 'Reign with Josh Smith' podcast: "For me, definitely in my early 30s to now, the thing has been opening up to people like my friends.
"I know it sounds silly but I had never really understood that love and protection was by being honest and being awful, like showing your worst side of yourself and then having someone go, 'Oh yeah, me too.''
Earlier this year, meanwhile, Claire confessed that she feels under pressure as a mother.
The actress - who has Ivy, six, with her ex-husband Stephen Campbell Moore - said: "There's this pressure to be this cake-baking, fun, playing 24-hours a day mother, being some sort of vehicle for entertainment, love and food.
"I'm just prepared to apologise for who I am: 'I am so sorry – but you're lumped with me. This is the hand you've been dealt, let's try to make the best of it.'"
Listen to the podcast at https://podcasts.apple.com/gb/podcast/reign-with-josh-smith/id1549676420.