Cressida Bonas feared her ambitions would be "trampled" if she stayed with Britain's Prince Harry.
The 25-year-old beauty split from the 29-year-old royal in April after almost two years together and friends claim she felt she had to end the relationship or give up her burgeoning acting career.
A source told the Daily Mail newspaper: "She sees her future as an actress and she felt her ambitions would have been trampled by the royal machine. The decision not to continue the relationship came from her, not Harry.
"The time had come for someone to make the break, and she could not see a long-term future with him."
Despite their split, the former couple - who were introduced by Harry's cousin Princess Eugenie - are still in touch, but only as friends.
The source added: "Harry was still very keen, but he appreciates her position and they have kept in touch -- as friends."
Insiders claim Cressida grew tired of Harry's "needy" behaviour, which included him inundating her with texts and phone messages "to the point where it became almost unbearable".
A source added: "He would be petulant and sulky, demanding she come round to see him all the time."
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