@JoanRivers / Twitter 1 / 2
@JoanRivers / Twitter 1 / 2

Kelly Osbourne, like so many other celebrities and fans, is mourning the loss of the legendary comedian Joan Rivers. The reality star wrote a heartfelt message to her late E! "Fashion Police" co-star just hours after she passed at the age of 81 via Facebook on Thursday, Sept. 4.

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"I'm completely heartbroken by the loss of my beloved Joan. Not only was she my boss, she was and will always be my teacher, therapist, closest friend, inspiration and the only grandmother I ever knew," Osbourne, 29, wrote, alongside a picture of the two on the Fashion Police set. "She was family and I will never forget her. Laughter will be difficult for a while but when I'm sad, lonely or upset all I will have to do is think of Joan and a smile will cross my face. Laughter is what she gave us and laughter is what she would want us to do in remembrance of her."

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She continued: "Melissa and Cooper, my thoughts are with you and I love you both so much. Joan, thank you for taking me in and loving me as a daughter. I will miss you deeply and will always hear your voice in my head saying 'my darling get out there and be you!'" (Osbourne also tweeted that her life "will never be the same" without the star.)

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Osbourne worked on "Fashion Police" with Rivers since 2010. Rivers opened up to Us Weekly about her connection with Osbourne and how she saw her as a surrogate daughter in February 2013. 

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"I adore her and I'm very protective of her. And it has been wonderful to watch her, she's grown on our show, and she’s changed and grown and matured and figured out the business," Rivers said at the time. "Kelly not only asks [Melissa] advice but do you want to know what's wonderful? She listens," she added. "Kelly is wonderful."