Things went south last week for the couple during a vacation on Virgin Gorda, the third-largest of the British Virgin Islands -- with Swift cutting the holiday short and jetting back home to Nashville.
"They had a fight," one insider tells Us. British crooner Styles, 18, "said something he shouldn't have . . . This [breakup] is definitely going to be for good."
"They are 100 percent over," another insider adds, noting that the "Red" chanteuse could be "intense."
Swift was first seen flirting with Styles back in mid-November -- and was photographed holding hands with the boy bander a couple weeks later. Before ringing in the New Year with a sweet kiss, they went on a ski trip with another volatile young couple: Justin Bieber and Selena Gomez .