The singer-songwriter, 30, had been with The Hunger Games star Hemsworth on-and-off for almost a decade after meeting on the set of the 2010 romantic film The Last Song, before tying the knot in December 2018.
The marriage came a month after California wildfires destroyed their Malibu home, and they split less than a year later.
Former Disney Channel star Cyrus appeared on her new Used To Be Young TikTok series, in support of her new single of the same name, to specify details about the day she knew that her relationship was over.
She said: “So Glastonbury was in June, which was when the decision had been made that me and Liam’s commitment to being married just really came from, of course, a place of love first, because we’ve been together for 10 years, but also from a place of trauma, and just trying to rebuild as quickly as we could.
“The day of the show was the day that I had decided that it was no longer going to work in my life to be in that relationship.”
The singer described it as “another moment where the work, the performance, the character came first”.
She said her priorities have since changed and she is now focusing on making sure the “human comes first”.
During her 2019 performance, Cyrus credited Glastonbury with changing her life in an expletive-laden speech during her set.
The Pyramid Stage set saw Cyrus joined by Lil Nas X and her dad, Billy Ray Cyrus, for a performance of Old Town Road, as well as to perform tracks Jolene, Party In The USA, Mother’s Daughter and Wrecking Ball.