This is my cat Belle, this specific photo is about 3 years old. She is currently 17, but today had been diagnosed with very aggressive lung cancer. She will not make it to Christmas, and we have decided to put her down tomorrow to spare her the pain of what is to come.
I don't know why I'm posting this. She's spent the whole day with me in my bed since coming home from the vet. She hasn't left once, not to drink, not to eat, not at all. Her breath is becoming more labored, and a part of me wonders if she'll make it through the night. I suppose that doesn't really matter anymore.
This happened to me 2 years ago, so I know it's really really tough. We put our old boy Stanley down on Christmas Eve and it was devastating. He was a long haired tuxedo that looked almost exactly like your sweet Belle.
I hope you take comfort in knowing that her last moments will be right by your side, where she likely feels the most at peace. And I also hope that you take care of yourself, take the time to grieve, and lean on your loved ones when you can.