Tancock's then fiance knew what he was getting into when he proposed to her.
"In life, you don't know who you will fall in love with," he says. "But I would rather something short, and be as happy as we can be, than a lifetime of just going through the motions."
But it's still difficult to watch her pained breathing. "It's like she's suffocating, drowning," says Stuart Tancock. "You don't want to see anyone suffer, especially someone you love. If I could take one day and be her -- so she could get a day off."
Two days before the wedding, Tancock awakes and her lips and ears are blue -- her face is ashen gray. She has a temperature, which means infection.
"She can't breathe," says Stuart Tancock. "This has never happened before."
Her doctors encourage Tancock to have her wedding in the hospital, but she refuses. Against their advice, the wedding goes on and three CF nurses accompany her in case of an emergency. Tancock is hanging by a thread.
Stuart Tancock wipes away his tears as he pays tribute to his courageous bride. A silver satin bag made by her aunt discretely hides her oxygen tank. "She's got drive and determination like no one else in the world," he says. "That's why I'm marrying her."
Eight days after the wedding -- and after three months on the transplant list -- the couple gets a call that finally lungs are available for transplant. But after Tancock is rushed to a specialized hospital in London, doctors discover the organs are too big for her body.
Doctors say she needs a miracle to live. Two weeks later, her miracle arrives; another set of lungs become available and they are a match.
Tancock endures a four-and-a-half hour surgery. "I was just hours from death," she told ABC News, recalling the "intense and traumatic" pain. At one point, she even tried to get her family to shut off the machines that had been keeping alive.
"I was in and out of sedation and very sick," she said. "From time to time I came around and tried to communicate with people. I had given up, enough is enough, I had drawn a line."
Now, Tancock said she is healthy, "Fitter than I have ever been" -- though her recovery was long and difficult. "I couldn't hold my head up," she said. A tracheotomy had wasted away her vocal chords and she had to relearn to talk.
She will have to take anti-rejection medication for the rest of her life and needs antibiotics because her immune system is suppressed.
Today, she writes a blog, 2nd Chance at Life, and volunteers with organ transplantation and cystic fibrosis organizations in her native Britain. She is also training for a 180-mile bike ride in September for charity.
She agreed to participate in the documentary because, she said, "People don't get to see what living with CF is like." She hopes "Dying to Breathe" will raise awareness.
Tancock said she is not religious, but draws strength from a sense that there is some sort of afterlife. An experience while she was near death suggests she may be right.
"My friend passed away when I was 10, and it was her birthday that day," when Tancock almost died, she said. "She was 16 and my first connection to CF."
Debby died within weeks of receiving her lung transplant. And Tancock said her friend appeared as an apparition, just as she was ready to give up. "It was her, telling me to keep holding on."
The battle isn't over, even though her new lungs may give Tancock another decade or two of life. CF will continue to slowly ravage her body.
Somehow, she still manages to stay positive.
"I am lucky even if I get just one more day of life out of this," she said. "At the same, there can be complications -- this is not a cure. I could reject the lungs tomorrow. Nothing is guaranteed, but I try my best and do what I can."
She added, "Today, I can be who I want to be."