Very ill dog- how to cope with death?

My dog is really sick. She is an old dog and developed arthritis and bone cancer. Everyday she is getting weaker and weaker, and it has come down to the end. She hasn't eaten in 3 days and hasn't drank water in around 2 days. If she doesn't p away naturally, my family decided it is best to consider euthanasia. She has labored breathing but is still some-what attentive. We will have to put her to sleep within the next day or two because she is so frail and sick. I'm absolutely heartbroken and don't know how to deal with losing her. I'm now 20 and have had her since I was 9 or 10. She is a huge part of my family. I'm devastated and can't stop crying. Does anyone have any suggestions to help me cope with her death?