The 23-year-old singer based her album 21 on the split from the unnamed man, and said that looking back she can see what went wrong.
'It was the most brilliant relationship I have ever been in,' she told Now magazine.
'I expected too much, I was needy, I was jealous even though I'm not a jealous person. We both said a lot of things in the heat of the moment.
'We were perfect and we were totally meant to be together - we just met at the wrong time.
'Maybe we'll be together in the future, I don't know.
'I'm constantly looking for him in other people.'
And she revealed that the tears at the end of her track, Someone Like You, are actually real tears shed for her ex.
'On Someone Like You I am really crying,' she said. 'You can hear it towards the end.'