It was exactly three weeks ago that ABC News' Amy Robach underwent a double mastectomy to treat the breast cancer that was discovered in October when she had her first-ever mammogram live on " Good Morning America."

Today, Robach returned to "GMA" for the first time since her surgery and said that, for her, undergoing that on-air mammogram was "the difference between life and death."

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"I had had that prescription for a full year and I didn't go and, you know, cancer spreads," said Robach, 40. "With the FDA regulations that woman can wait until they're 50, it was my excuse to say, 'Eh, I can wait a couple of years, if some doctors say 40. Some people say 40. Maybe I'll split the difference.'"

"Well that probably would have meant the difference between life and death for me," she said.

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Robach said on "GMA" that while undergoing the mastectomy, her doctors discovered problems in her left breast and also found the cancer had spread to her sentinel lymph node, which means she will need to undergo four months of chemotherapy.

"That was the devastating news to me," Robach said. "When I woke up, the first question I asked was, 'Did it spread to my lymph nodes?,' because that's the difference between chemo and no chemo."

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Robach begins chemotherapy Dec. 16 but said she plans to continue working, following the model of "GMA" co-anchor Robin Roberts, a breast cancer survivor who had urged Robach to get the on-air mammogram Oct. 1, "GMA" Goes Pink Day.

"You give me strength, Robin, because it is important to get up and have something to do each day, even if you don't feel great," Robach said. "Some people have it harder than I do so I will take it hour by hour."

Robach, the mother of two young daughters, said she is also following the advice Roberts gave her early on after her diagnosis, to take things hour-by-hour and day-by-day.

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"What it forces you to do is live in the moment and I think that's a good lesson for everyone," Robach said. "There are days where if you really start to let your mind wander, it's fairly devastating. But, you keep your head together. You come into work. You hug your friends and you're thankful to be where you are with your family and the people who matter."

Robach was surrounded by the people who matter most in her life, including husband Andrew Shue, this Thanksgiving, the first holiday since her diagnosis and one she described as "very special."

"When we all went around and said what we were thankful for, you know, we usually say whatever happened that year," Robach said. "I uttered words I never thought I would utter, 'I'm thankful to be alive.'"

"It's good to know that you get to be thankful to be alive," she said. "It's a reminder to all of us that you never know what's around the corner."