-- Although she's just 6-years-old, this might be Addie Fausett's last Christmas.
"She's loved Christmas," Addie's mother Tami Fausett, 29, told ABC News. "It's been really hard."
Last year, an MRI revealed that Addie had extensive atrophy to her brain.
"[Doctors said there was] roughly a year for her left, and that could go sooner if the wrong part of the brain decides to go," said Fausett.
When Addie was 3-years-old, the stay-at-home mom from Fountain Green, Utah, noticed that her daughter stopped growing.
"We had taken her to the doctor because we noticed her behavior started to change and she was kind of shaky with her hands," Fausett said. "She started to slow down a little bit. When she was closer to 4, she fell off the growth chart."
Despite multiple doctor visits and numerous tests, doctors have been unable to determine what exactly is ailing Addie, who has never weighed more than 25 pounds.
"It angers me that they can't figure out what's wrong, what's taking my little girl," said Fausett, who quit her job to take care of Addie. "I know even if they found out what it was, they wouldn't be able to stop it. I would just love to know what it was, because it's hard to know your kid's sick but to not know what it is."
With Christmas just weeks away, Fausett said Addie is having a harder time with talking and doing a lot of things.
"She's a happy kid, but a lot of days she just cries all day," Fausett said. "I don't know if she's just in pain or just, you know, having trouble communicating or just doesn't feel good."
Since Addie is unable to play with other children because of her restrictions, Addie's grandmother came up with the idea to ask people to send Christmas cards to Addie and her sisters, Shayley Fausett, 10, and Audree, 7, and to tell Addie they were her friends.
Every day, the family and Addie, on her good days, pick up hundreds of Christmas cards at the post office.
"She loves it. She just smiles," Fausett said. "And some of them are her boyfriends. One little boy put that he was her boyfriend, and another little one she said, 'This one is my boyfriend.'"
With the help of family and friends and social media, Addie has received cards from all over the world, including Saudi Arabia and Germany.
"I'm hanging them on the walls. I've got to come up with another way. My house is not big enough," Fausett laughed.
The cards are a comfort to Addie and her sisters, who are also dealing with the recent loss of their father and Fausett's estranged husband, who passed away on Nov. 29.
"I don't know how to thank everyone. It's amazing to me that they're so giving to somebody they don't even know," Fausett said.
"It's not a lot, but that card means so much to Addie."
If you'd like to send a card to Addie, you can mail it to:
Addie and Her Family
P.O Box 162, Fountain Green, Utah, 84632