The 35-year-old took to Instagram yesterday to reveal her heartbreak, as Jack allegedly went public with his new girlfriend as Caroline prepared for the 'biggest day of her life'.
Last night The X Factor aired the first episode of the new series to a small audience in central London, with Caroline making her debut as co-host of the main show.
Sharing a snap of her holding a disco ball, Caroline wrote: "When someone who knows you more than anything ruins the biggest day of your life [sic]."
Caroline and Jack dated for 18 months before splitting last year, and they reunited briefly in June.
A source told the Mirror: "Caroline is heartbroken to see him move on so fast.
"She thought they could make a go of it the second time. Clearly it's over now. This break-up has been even more acrimonious than the last. And the timing of him going public with his new girlfriend couldn't have been worse.
"She has been gearing up for the X Factor launch for months. All she wants is for it to be a success and she felt she didn't need the drama."
It's thought that a snap uploaded by Sam Smith – who Jack manages – showing him getting close to a mystery woman in Ibiza was confirmation enough for Caroline.
Thankfully Caroline had her co-host and bestie Olly Murs to cheer her up. Following the launch, she shared a snap of the duo on the red carpet, adding: "When your friends are brilliant … X [sic]."