The Xtra Factor presenter, who split with Styles in January, admitted that she found the media speculation over their romance "really tough" to deal with.
She told The People: "Everyone had an opinion on me and Harry, which was the hardest thing.
"I suppose they're entitled to their opinions but it was really tough. I learned the hard way.
"I learned a very difficult lesson but I suppose looking forward it isn't a bad lesson to learn, is it?"
The 32-year-old added: "In future the easiest thing is that I keep it to myself. I can't believe how much attention there was on me."
Fellow celebrities Kerry Katona and Rebecca Ferguson both questioned the ethics of the relationship, given the One Direction star is 15 years Flack's junior.
> One Direction's Harry Styles: 'I'm through with older women'