The mother-of-one, 36, revealed she had separated from her long-term partner on her Instagram Stories, where she made the revelation in an Ask Me Anything "uncensored" post.
Responding to the question "Are you and Derek still a couple?", she broke the news to her fans. "Nope," she said. "I am single momming it and co-parenting."
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Despite her recent heartbreak, Rumer exuded confidence as she attended the star-studded soirée at Hangar Nicelli in Venice, Italy.
Her mesmerizing dress featured a fitted corset that cinched her waist, along with a billowing cape and swathes of rich brown fabric draped across her hips before cascading into a rippling skirt.
Rumer's platinum blonde hair couldn't be more different from her mom's famous raven locks in color, but in length, the mother-daughter duo both sport waist-length locks.
For makeup, the Once Upon A Time In Hollywood actress rocked an edgy smokey eye and nude lips to draw attention to her striking features.
In a recent Instagram post, she referred to her mini-me as "my soulmate and the love of my life," adding: "The last year has been so wild and challenging and beautiful and you are such a light. I am so grateful for you. For your tenderness and love. I learned so much this year. So much about myself."
She continued: "I feel more free and happier than I have been in a long time. Navigating through this last year has shown me strength inside myself I didn’t know was possible and I am so proud of myself."