"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, although the two 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." A rep for Cyrus had no comment.