The couple, who went public with their romance in September after releasing their duet 'Almost Is Never Enough,' broke up a few weeks ago as their relationship had run its course.
A source told UsMagazine.com: ''They just weren't into it anymore.''
The 20-year-old 'The Way' hitmaker previously gushed about the ''amazing'' The Wanted singer, 20, and recently insisted it wasn't ''tough'' for them to spend time together despite their busy schedules.
She told the February 2014 issue of Teen Vogue: ''It's not as tough as people would think. We have the same music manager, so we can see each other's schedules and plan little trips as often as we can.''
But a source close to Nathan said they ''hardly ever saw each other'' and are currently on a break.
Ariana hinted all was not well between them last week during a radio interview with Florida's 93.3 FLZ.
When asked about their relationship, she said: ''You're a little behind. We're really good friends.''
This Doctor won't be saying 'goodbye' in the same way as those who have left the series in recent years.