The social media influencer, who had been in a relationship with Payne for around two years, said she was at a “complete loss” after police in Argentina said the singer died falling from the third-floor balcony of a hotel in Buenos Aires.
“Thank you for all of the kind words and love that has been sent my way,” Cassidy said in a statement posted on her Instagram story.
“I have been at a complete loss. Nothing about the past few days have felt real.
“I ask and pray that you’ll give me the grace and space to navigate this in private.
“Liam, my angel, you are everything. I want you to know I loved you unconditionally and completely.”
Cassidy said she will continue to love Payne “for the rest of my life”, adding: “I love you Liam.”
Her tribute came moments after music mogul Simon Cowell said he is “truly devastated” after the death of the singer, in a statement posted on Instagram.
Payne found fame alongside Niall Horan, Louis Tomlinson, Zayn Malik and Harry Styles when The X Factor creator Cowell put them together to form One Direction on the ITV talent show in 2010.
“You never really know how you feel about someone until a moment like this happens,” he wrote.
“Liam, I am truly devastated. Heartbroken. And I feel empty.
“And I want you to know how much love and respect I have for you. Every tear I have shed is a memory of you.”
Cowell said the singer’s seven-year-old son, whose mother is former X Factor judge Cheryl, will be “so proud” of what he achieved during his 31 years.
“I have met your son, Bear. He has your smile and that twinkle in his eyes you have,” Cowell wrote.
“He will be so proud of everything you achieved. And how you achieved it.”
The 65-year-old titan of the music industry said he would tell the thousands of fans that Payne was “kind, funny, sweet, thoughtful, talented, humble, focused” and “how much love you genuinely had for the fans”.
Payne first auditioned for The X Factor in 2008 when he was 14, singing Frank Sinatra’s Fly Me To The Moon, with judge Cowell telling him to return to the talent show two years later.
In 2010 he impressed the judges with a rendition of Michael Buble’s Cry Me A River before being put with four other solo hopefuls at the boot camp stage of the competition – the group which later became One Direction.
Cowell continued: “I had to tell you when you were 14 that this wasn’t your time. And we both made a promise that we would meet again. A lot of people would have given up. You didn’t.
“You came back and within months the whole world knew Liam. And you never forgot the fans.
“I watched you spending so much time with people who had wanted to meet you. You really cared.”
Cowell said that Payne had visited him last year “not for a meeting, just to sit and talk” and they “reminisced about all of the fun times we had together”.
“After you left, I was reminded that you were still the sweet, kind boy I had met all of those years ago,” he said.
Cowell concluded that he always felt the five boys in One Direction were “brothers”, adding that after their separate tributes on social media “I believe you were”.
It comes after Horan was the last bandmate to share a touching tribute, saying he will “cherish every moment” he had with Payne “forever”.
Payne was spotted dancing at Horan’s show around two weeks ago at the Movistar Arena alongside his girlfriend Cassidy in Argentina.
“I feel so fortunate that I got to see him recently. I sadly didn’t know that after saying goodbye and hugging him that evening, I would be saying goodbye forever. It’s heartbreaking,” Horan wrote.
Payne’s other bandmembers also paid emotional tributes to him while a joint statement from the band said they were “completely devastated” and will miss the singer “terribly”, adding the “memories we shared with him will be treasured forever”.
The singer’s family said they were “heartbroken” following his death, adding: “Liam will forever live in our hearts and we’ll remember him for his kind, funny and brave soul.”
The singer died of multiple traumas and “internal and external haemorrhage”, a post-mortem examination report said.
Argentina’s National Criminal and Correctional Prosecutor’s Office No 16 said it was investigating the incident as an “inconclusive death” following the report.