Bachelorette Katie Thurston says she went from wedding planning to battling cancer

Former Bachelorette Katie Thurston is opening up about being diagnosed with Stage 3 breast cancer at age 34.

"You're so devastated. It's so shocking," Thurston told ABC News' Eva Pilgrim in an interview that aired Wednesday on "Good Morning America." "As a 34-year-old woman, it's just, you're not prepared."

Thurston, who starred in season 17 of "The Bachelorette" in 2021, said the road to her diagnosis began when she noticed a small lump on one of her breasts, a symptom of breast cancer that she said she initially dismissed.

"When I felt my lump, it was sore, and as women were used to being told like, 'Oh, it's it's probably their time of the month,' or 'Oh, it's from working out,' so I brushed it off for a while," Thurston said. "And then finally I decided to go to the doctor."

After seeking medical care, Thurston said she was diagnosed with Stage 3 triple positive ductal carcinoma, a type of cancer where tumor cells have estrogen receptors, progesterone receptors, and HER2 receptors on their surface, according to the U.S. National Cancer Institute.

For Thurston, the diagnosis meant she would need to undergo chemotherapy before she could have the cancerous tumor removed.

"You're on, like, survival mode. Like, you don't have a choice -- you show up, or you die," Thurston said of her experience being quickly thrust into the world of breast cancer treatment and appointments. "And I hate to say it that way, but, like ... every day you have an appointment, every day you have to show up. You don't have a choice."

Thurston received her diagnosis in February, just a few months after she became engaged to comedian Jeff Acuri. Instead of planning their wedding, she said she and Acuri began planning for their future in a different way.

"We thought we'd be planning a wedding. We thought we'd be trying for a baby," she said. "And instead, we know that this year is really dedicated to just treatment and getting better."

Thurston said she chose to harvest her eggs following her diagnosis in case she and Acuri try to have a child together in the future.

"We've done everything we can to be, you know, proactive for our future and our family plans," she said, adding that she and Acuri are also OK if their proactive steps ultimately don't work. "But in the event that IVF didn't work out, in the event that chemo did impact me in a negative way when it comes to my reproductive health, we're also OK being a childless couple."

Thurston shared her breast cancer diagnosis publicly in an Instagram post on Feb. 15, writing that she found support and hope after her diagnosis in the stories of other women who also faced breast cancer.

Thurston told Pilgrim that she hopes sharing her story encourages people to be proactive about their own breast health.

"I think that's the biggest takeaway, is telling people like, 'Don't wait,' you know, 'Be proactive, get checked out,'" she said. "You could be doing yourself a favor in the future."

Most women with average risk should start screening for breast cancer at age 40, and get screened every other year through age 74, according to guidance from the U.S. Preventive Services Task Force, a federally appointed group of experts whose conclusions guide medical decision making and determine some insurance policies.

Breast cancer screening tools include a mammogram, which is an X-ray of the breast.

Dense breast tissue may make a mammogram more difficult to interpret, and may make detecting cancer more difficult, according to the National Institutes of Health. Women with dense breasts may be called back for follow-up testing, including ultrasound and/or magnetic resonance imaging.

Women at higher risk of getting breast cancer may also receive an MRI scan.

Throughout their lifetime, women should also watch for new lumps, skin changes and nipple changes, including unusual discharge, pain, and redness.