"This isn't how the story was supposed to go," she wrote on Internet.
The Golden Buzzer singer, who withdrew from the talent competition in August after her cancer battle took a "turn for the worse," opened up about missing the finale. Magician Dustin Tavella was ultimately crowned the champion Wednesday.
"It's so hard for me to not be on the agt stage for the finals this week," wrote Nightbirde, whose real name is Jane Marczewski. "I bet you never saw someone win so hard and lose so hard at the same time."
She continued: "I'm raging and crying and hoping with everyone who needs to rage and cry and hope tonight."
The Ohio singer captured America's heart during her emotional performance of her original song "I'm OK,"which highlighted her fight with cancer. During her audition that aired in June, Marczewski said she has a "2% chance of survival" with "some cancer in my lungs, spine and liver… But 2% isn't 0%. Two percent is something." (Her audition performance has over 37 million views.)
At the time, Marczewski told the judges "you can't wait until life isn't hard anymore to decide to be happy." Cowell choked up and said her song "really got to me."
Cowell hit his coveted Golden Buzzer, which automatically sent Marczewski to the live shows that started in August, but the singer announced that her "fight with cancer is demanding all of my energy and attention."
"I am so sad to announce that I won't be able to continue forward on this season of AGT," "Life doesn't always give breaks to those that deserve it — but we knew that already."
Although Marczewski couldn't participate in the finals, Cowell had no doubt that she would have made it all the way to the end. "I know she would have been at the finals tonight. We're are all sending her our love tonight," he said during Tuesday's finale performances.
Cowell told People he speaks to Marczewski "once every two weeks" and the British judge regularly shared health updates with viewers. On Tuesday, he said, "She's doing better."
In August, Marczewski virtually returned to "AGT" to thank viewers for all the love.
"I cannot say thank you enough for the opportunity to have a moment to share my story and we are witnessing such a beautiful picture of the human spirit," she said, adding that the overwhelming amount of support she received has "restored my faith in humanity."
Cowell said Marczewski made the right decision to focus on "your well-being, your health and your recovery."
"You made the decision, rightly so, that your health is your priority right now and I know the last time we spoke you actually said that you feel like you let people down," Cowell told her, holding back tears. "On behalf of all of us, you haven't let anyone down. Even though you haven't competed – you already won."