The former 'The Wanted' member - who dated the 'Problem' hitmaker for four months in 2013 - burst into tears in the studio while recording a track about his ex.
Speaking to the Daily Mirror newspaper, the 22-year-old star revealed: ''It was quite an emotional day. I found that especially when you go over the lyrics again. It's like, 'Wow! Those are really deep lyrics.' I got really emotional writing it.
''I think three or four songs were really emotional to write.
''There's one song called 'Famous' which is about what do I have to do to make someone I care about, care about me as much as I do them? It's basically saying: do I have to be famous, will you only call me when I'm famous? Whenever I go through something that's not ideal, I don't feel sorry for myself. I go, 'My God, this is going to be such a great song'.''
The 'Kiss Me Quick' singer showed no hard feelings towards the brunette beauty though as he thinks she's an ''incredible talent''.
He told the Daily Star newspaper: ''It's fantastic she's doing so well because she is such an incredible talent.
''I've known her quite a while and I know how much she's always wanted a Grammy nomination, so she must have been so happy to get one this year.''
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