Kristina Rihanoff said that when Caroline Flack won Strictly Come Dancing in 2014, there were "demons" behind her smiles.
The former Strictly professional paid tribute to the Love Island presenter and revealed that while on the BBC dance show, Caroline was battling "a lot of insecurities" at the time.
A lawyer for the Flack family confirmed that Caroline took her own life and was found in her east London flat on Saturday.
Kristina became close friends with Caroline while she was on Strictly and speaking on Good Morning Britain on Monday, she described Caroline as "sweet" and said that she enjoyed working with her.
Kristina said: "She had a lot of insecurities and vulnerabilities not known to the public.
"We all knew her beautiful smile but behind closed doors, there were a lot of demons."
The pro dancer said she was "heartbroken" that Caroline "couldn't see a way out" and sent her condolences to the Flack family.
The Love Island host had previously said that she fell into a year-long depression after winning winning Strictly.
This time saw Caroline "battling inner conflicts" and Kristina said that the Strictly winner had missed a number of performances during the subsequent tour because of it.
Caroline was reaching peak fame as the host of ITV2's Love Island, which was proving one of the most popular reality shows on TV.
She stepped down from the hosting role ahead of the new winter series, after she was arrested for allegedly assaulting her boyfriend Lewis Burton.
Caroline was due to stand trial on March 4 after pleading not guilty to assault by beating following the early-morning altercation with her boyfriend.
Her death reportedly came just hours after she found out that the CPS would go ahead with the trial.
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