'The Bachelorette' Star Katie Thurston Announces Breast Cancer Diagnosis

Katie Thurston's sharing a major life update ... telling her fans she's been diagnosed with breast cancer -- and, she plans to share every bit of it.

The 'Bachelorette' star shared the news in an Instagram post Saturday morning ... telling fans she and her fiancé, comedian Jeff Arcuri, planned to travel the world before settling in NYC -- but, they had to cut it short so she could deal with getting medical coverage for her illness.

She says her recent life has run her through a gauntlet of emotions ... from anger, sadness to denial -- before she emerged on the other side ready to fight cancer.

Katie says she looked into stories from other people who have been in the same boat as her ... and, she says it's encouraged her to share her own journey -- even though it's going to be difficult.

She ends her message by shouting out Arcuri ... telling him she doesn't know where she'd be without his selfless nature -- adding "The selfless love that you smother me in is beyond anything I imagined I’d be blessed with. I love you to the fullest in this lifetime and the next."

Katie was a contestant on season 25 of "The Bachelor" ... and, though she didn't wind up with that season's leading man, Matt James, she was a fan favorite who got her own season of "The Bachelorette."

She ended up accepting Blake Moynes' proposal ... though the two ended their relationship months after filming wrapped.

Katie and Jeff met just last year ... but, they instantly connected -- and, he proposed on a beach just months later.