Witherspoon, who stars alongside Winfrey in the upcoming Disney film "A Wrinkle in Time," said that the relationship was psychologically and verbally abusive, and occurred when she was very young.
Making the decision to leave her partner, whom she did not name, was "profound," she added.
"It changed who I was on a cellular level, the fact that I stood up for myself," Witherspoon said. "I didn't have self-esteem and I'm a different person now. And it's part of the reason I can stand up and say, 'Yes, I'm ambitious,' because someone tried to take that from me."
Last October, as dozens of actresses came forward with allegations of sexual misconduct, Witherspoon, 41, spoke at Elle's Women in Hollywood event about being assaulted as a teenager by a director. She said that the incident was not an isolated one and praised her colleagues for telling stories that they'd previously kept quiet.
"Hearing all of the stories these past few days and hearing these brave women speak out tonight about things we’re kind of told to sweep under the rug and not talk about, it’s made me want to speak up, and speak up loudly," she said at the time. "I felt less alone this week than I’ve ever felt in my entire career."
Now, Witherspoon is working to tell stories of complex women through projects like her HBO series, "Big Little Lies," and hopefully, inspiring others to feel comfortable defending themselves and their ideals. The word ambition, she noted, is not seen as being synonymous with being feminine, and that, she added, should change.
"[Ambition] isn't about being selfish. It isn't about being self-serving. It's actually about wanting to create more and do better for communities," she said. "An ambitious woman is not a terrifying thing."
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