"They are both really busy and never in one place for long," explains a source to L&S. "Harry's really upset about the split and is really sensitive about it. Loads of his friends told him to be careful and that it was never going to last, and he feels a little foolish now that it's fallen apart so quickly."
Styles returned to London on Monday, guitar in hand, amid screaming fans and snapping paparazzi.
The split reports come less than two weeks after the emergence of Haylor, who were first spied holding hands during an "X Factor" rehearsal in Los Angeles back in November.
Swift and Styles confirmed the romance with a lovey-dovey photo op at New York's Central Park Zoo (left) and several implied hotel sleepovers.
The boy-bander reportedly met her parents following a One Direction performance at Madison Square Garden, and she celebrated her 23rd birthday with his family (and cupcakes) as he showed her around northern England.
So, as we await the official word on whether Swift is furiously composing songs about her seemingly brief entanglement with Styles (imagined lyrics: "There's only one direction for you to go/and that's away from me"), take a look back at photos of them together.
And remember, he'll always have his "I Knew You Were Trouble"-inspired bicep tattoo, and we'll always have their "Dirty Dancing" lift …
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From Crowd Ignite