The Crown star reveals she suffered a mental "dip" at the beginning of the COVID-19 lockdown when her career ground to a halt, shortly after attending the Screen Actors Guild Awards at the start of the year (20).
She told Grazia magazine: "We'd just finished filming the fourth season (of The Crown) the week before everything shut down.
"The brakes got put on everything and my mental health did dip. As an actor, I didn't have a purpose. I'd gone from zero to 200 miles per hour and then back to zero. I glimpsed red carpet life and then it was gone again."
"To sit in my own thoughts was a difficult process at first, because you tune in with where your body and your mind and your soul and your spirit are," she continued.
"Part of that process is going, 'Right, I really need to look at this because I don't feel happy about that' or 'I really haven't addressed these issues that may have been there since childhood.'"
Erin soon found that picking up a new hobby did wonders for her mood.
She said: "I learned to juggle. It's kept me sane. I just needed to focus on something."
The British star has since also had to adjust to the fact she'll no longer be playing Princess Anne in the hit royal drama series, as she passes on the role of Queen Elizabeth II's only daughter to a new, as-yet-unannounced actress for season five.
"It's the biggest shock," she confessed. "I said it out lout for the first time only a few weeks ago. I had the breakthrough of my body and brain connecting that I'm not going to play this woman any longer.
"I realised I had to let go of her. It's a sad thing."
And although Erin feels "really weird" about someone else taking over the role for the next two seasons, she's "genuinely excited" to watch the show again.
"I want to watch the show," she added. "I haven't been able to watch it for two years now, because I just can't watch me.
"I'm excited to hand over the wonderful gift that is Princess Anne and let someone else roll with it."
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