By Char Adams
Updated February 11, 2016 02:30 PM
Burns Family

In the days since Brittany Burns’ death, her family members are leaning on one other for support.

“There’s no playbook for this. We’re taking one step forward and we’re leaning into each other,” Cathy Burns, Brittany’s mother, tells PEOPLE. “Cancer won this time. But we don’t want cancer to win in the future.”

Brittany, the fiancée of Buffalo Bills linebacker Tony Steward, died earlier this month after a brief battle with cancer. In December, she was diagnosed with a rapidly-progressing, rare form of ovarian cancer, and passed away less than two months later.

Her death came as a shock to the 26-year-old’s family and fiancé. In October, she had gone to a doctor complaining of stomach pains and fatigue, but what officials thought were simple ovarian cysts, turned out to be the rare, terminal cancer – a combination of carcinoma and mixed germ cells.

“[The doctor] told me, in all the years he had done this, he had never seen a cancer move this fast,” Ty Burns, Brittany’s father, tells PEOPLE.

Brittany and Steward moved to Buffalo, New York, after he was drafted by the team in 2015. But the two were hardly settled before the illness prompted Burns to move back to Delaware to be with her family.

“We were praying that the chemo would beat the cancer,” Cathy says. “Every now and then we’d get a sign that was happening even toward the very end, we never gave up.”

The Burns say they sought the best medical treatment for their daughter.

“Not only were we trying to catch a cancer that was very aggressive from the get-go, but the chemo was so that [the doctor] had to respond to how her body was responding,” Cathy says. “That’s when we found that the original chemo treatment wasn’t working the way we needed it to.”

Still, Ty says Brittany battled hard against the disease.

“I could not believe how strong she was and how hard she fought. It was unbelievable, the pain that she dealt with,” he tells PEOPLE. “The cancer got into her bones, into her liver, but she persevered beyond anything that I could have imagined.”

Cathy adds: “It was incredibly heartbreaking to watch Britt endure an unbelievable amount of pain. I have never seen someone fight as hard as Britt did. She lived life, she embraced life and she fought for her life until the very end.”

On the day Brittany died, her hospital bed sat in the living room and she was surrounded by her parents, Steward and her siblings.

“What I remember most about the last several hours is our family was together,” Cathy says. “There was no doubt that Britt knew how much all of us loved her. As complete as you can be with something as tragic as this.”

The family, as well as hundreds from across the country, gathered for Brittany’s funeral service on Saturday. Ty says there were more than 400 people there, including more than 50 rowers from Clemson University – where Brittany was on the rowing team – and a few of Steward’s NFL teammates.

“As her dad, I miss my daughter. I miss my little girl. It still doesn’t seem real. No question, this is the darkest moment in my life,” Ty says. “I’m surrounded by my family, which is great, but there’s nothing anybody can say or do that makes it okay.”

Cathy adds, “This has been the deepest pain I’ve ever felt in my life. Britt was my step-daughter. I treated her and felt like she was my daughter. She usually calls me ‘Cathy,’ and all day Sunday, she called me ‘mom,’ ” she says, recalling the day of Brittany’s death.

As for Brittany’s siblings – Maia, 12; Alexis; 16; John T, 36 and Troy, 39 – Ty says they are struggling more than he’s ever seen.

“The younger ones are trying to understand it, and the older ones are trying to cope with it,” he says.

In the midst of their grief, the Burns family has set its sights on raising awareness about ovarian cancer and raising funds to research the disease – Cathy say it is a “silent killer.”

Brittany and Steward started the effort, launching “Britt and Tony’s Fight Like a Girl Campaign” to raise money to research the disease. And the Burns vow to carry on her legacy.

“Cancer won the battle, we’re gong to win the war,” Cathy proclaims. “We have the platform and commitment to do that.”

The campaign has raised nearly $50,000 and the Burns say they’ve received a lot of support from the Buffalo Bills and the NFL.

But they say they are hoping to bring in an even bigger donor so they can “help educate women and make them understand” the enigmatic disease.

“The more funding and research that we can get in to genomic testing, specific treatments for those specific genes, not only could it potentially prolong someone’s life, but get them into remission and ultimately cured,” Cathy says.

“Our commitment is to make a real difference,” she adds.

Cathy says focusing on the foundation is helping her through this tough time.

“I’m trying really hard to let go of these last six weeks. I’m shifting my focus to how I like my life going forward,” she says. “I don’t know what that means yet totally. A big part of it is the foundation, Britt and Tony’s foundation.

“I miss Britt a lot,” she says.