Cyrus, 16, ended her relationship with Gaston, 20, last week as she prepared to leave Los Angeles for two months to work on the movie "The Last Song," which is being shot in Savannah, Ga., an insider revealed.
Both parties are said to be upset by the split.
Cyrus's rekindled friendship with her ex-boyfriend, 16-year-old pop star Nick Jonas, reportedly caused a strain on Cyrus's relationship with Gaston.
"She obviously has been hanging out with Nick again a lot, and Justin was cool with it at first, as long as it was just a cool, friendly thing," a source told E! "She didn't cheat, but I guess Nick was trying to get her back the whole time."