Cressida Bonas split with Prince Harry after intense scrutiny of their relationship on social media and in public, according to a royal biographer.
Ingrid Seward, a writer who is also friends with Miss Bonas' mother, revealed the actress, 27, told friends she became fed up with the "awful" situation as she was continuously criticised, both in the street and online.
Miss Bonas, who is also a model, split with the Prince in 2014 amid speculation she could not cope with his highly public lifestyle.
Speaking at the Henley Literary Festival, Mrs Seward confirmed Miss Bonas told friends she split from the Prince after two years having been faced with a barrage of negative comments whenever they went out.
"She just said it was awful because every time she walked down the street she could hear people criticising her," Mrs Seward said. "They would whip out their phones and take photos of her. They'd say 'oh look at her hair', 'oh look at her clothes, look at her boots, she hasn't got her laces done up', I mean this is just a normal, pretty girl and she just couldn't take it. She just thought 'why are they getting at me?'."
Mrs Seward said the pressure of social media, which she described as "a kind of evil", was also significant for the Duke and Duchess of Cambridge, who are currently on tour in Canada.
The fear of putting a foot wrong had spread to the Duchess, she said, as she claimed the mother of two was concerned about doing anything "remotely controversial".
She also dismissed claims the Duchess is "boring", adding that she believes she is just "terrified". "Some [people] think she needs a little more personality but I think she is terrified," Mrs Seward said. "You wouldn't think she would be terrified because she has been around for nine years, but I think she is scared of saying anything that can be contrived as being remotely controversial."
Dickie Arbiter, the former press secretary for the Queen, who was speaking alongside Mrs Seward, added that the Duke and Duchess must be "very careful" when it comes to being caught on camera.
"Anything you do can be instantly reported on and pictures uploaded so you have to be very careful; any body movement can be misconstrued, a cough is bronchitis, a sneeze is flu, a scratch is scurvy."
He added: "There is a very great danger, particularly amongst the royals. They have to be very careful of what they do, what they say, where they are and how they do it because it can get reported on very quickly, it can very quickly go viral."
The biographer also revealed that Prince Charles was "extremely nervous" during Princess Diana's funeral procession, as he believed he may be murdered due to his status as "public enemy number one".
Mrs Seward said: "The streets of London [during the funeral procession] were very quiet. You could hear a pin drop. You could hear everything the crowd was saying. They were saying, 'Look at him, look at him.' They were being quite nasty.
"The whole way round he could hear this abuse and he didn't think he would make it the whole way round." Mr Arbiter said Prince Charles knew many people blamed him for the collapse of his marriage to Princess Diana.