The singer dated the One Direction star in 2010 when they were both booked on the X Factor tour.
Speaking to The Independent, Ferguson believes that the relationship may have stalled her career.
She said: "That was my first mistake. I was 23, he was 18. Of course, if it was a 23-year-old guy dating an 18-year-old girl, no one would have batted an eyelid, but this way around was a big deal, apparently.
"I didn't realize that. I was just a hippy, free-loving person, but that's not a good thing to be in this industry. You can't just follow your heart."
Ferguson was also shocked by recent comments in which Malik labelled the relationship a "mistake".
"I was so shocked by what he said! But I'm not going to retaliate", she said. "We don't talk any more. We're not in touch at all," she added.
Ferguson recently announced details of a 2014 UK tour. The singer will play ten shows across the country in March.