"Good Morning America" correspondent Amy Robach said Friday her breast cancer was worse than initially expected, but her prognosis is good. Robach, 40, announced two weeks ago she had been diagnosed with breast cancer after she had her first mammogram last month on "GMA." She is now recovering from a double mastectomy. "This message is long overdue. ... It has been weighing heavily on me how much gratitude I owe each and every one of you," Robach said in a letter posted on ABC News' website. "The unbelievable outpouring of support from ABC began from the first moment I received my diagnosis. Phone calls, emails, cards, flowers, blankets, care packages, food, pajamas, slippers, help with insurance, medical advice. ... The list goes on and on. ... "Physically and emotionally I have been through the ringer, but I am emerging on the other side so much stronger," she added. "I have a greater appreciation for life, for health and for how such simple acts of kindness can be so incredibly powerful. I am looking through a different lens now... and I am thankful for that as well. There was my life before Oct. 30 and now my life after. "My prognosis is good, I got very lucky finding the cancer through our ABC sponsored mammogram and I got lucky choosing an aggressive approach, bilateral mastectomy, because while in surgery last week my surgeon found a second, undetected malignant tumor. No MRI, no mammogram, no sonogram had found it... it was only through the mastectomy that she discovered it. My cancer had spread to my sentinel lymph node, but not beyond, so I will have more treatments ahead of me, but none that will take me out of work. As of right now, I plan to head back into the building Monday, Dec. 2 and I couldn't be more excited to get back to work."