"They have been having trouble for a while," says the insider. "It's been hard for a long time since Miley has been working so much and traveling a lot."
Contrary to reports, Cyrus' ex Nick Jonas "was not the reason for the split, but they have been spending time together."
Added the source, "Miley and Nick are working on a couple different projects together. Justin has been a little jealous of that."
The 16-year-old Hannah Montana star -- who recently flew to Savannah, where she will be shooting the upcoming Nicholas Sparks drama, "The Last Song" -- hinted at a split over the weekend when she told Us that Gaston, 20, won't be joining her on the road.
"He's got his own thing going on," Cyrus told Us. "But I am just continuing to work hard and definitely have the whole tour going on and my clothing line and the movie."