I went through the same thing 5 years ago, my wife's downward slide from cancer lasted 14 months. I was by her side almost all the time. I think I was up 36 hours straight when the end was near, I didn't want her to pass while I was sleeping and not be able to be by her side, I was there at the end tending to her.
My heart goes out to you, I understand what you are going through, I really do.