The 25-year-old singer opened up about her lingering feelings for Valderrama in her YouTube documentary, "Demi Lovato: Simply Complicated," which debuted Tuesday.
"I've never loved anybody like I loved Wilmer, and like, I still love Wilmer," Lovato confessed. "We connected on a level that I've never connected with anybody before. He was just my rock, my everything."
"I think my heart's always with Wilmer. I think it was with Wilmer, it is with Wilmer, and I think it will be with Wilmer, because you don't share six years with somebody and not give them a piece of your heart," she added.
Lovato and Valderrama announced their split in June 2016 after six years of dating.
The singer said in the documentary that the breakup was amicable and in part due to her desire for independence.
"It had nothing to do with falling out of love -- we decided together that we're just probably better as friends," she explained. "I just wasn't ready and there was so much in my life that I hadn't explored yet."
Lovato and Valderrama appeared friendly in June while reuniting at a barbecue at the singer's house.
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Lovato shared a photo on Instagram Stories with the actor at the time, calling him her best friend.
"Best of friends no matter what @wilmervalderrama," she wrote.
Lovato has dated Luke Rockhold and Guilherme "Bomba" Vasconcelos since her split from Valderrama.
She said in the documentary that she's "pretty sure" she will never meet anyone who compares to the actor.
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