I'm so sorry .
Your post brings back so many memories of my loss of Teddy three years ago. He was 6 and a half years old, it was the same time of year, and he had that same awful disease. In our case, too, we were initially told it wasn't cancer but it turned out it was and the pain progressed very rapidly until we had to say goodbye much sooner than we hoped.
It is utterly devasting and so unfair to lose Tawny so young and so unexpectedly. You will never be the same. Love always changes you. My life changed a lot after I lost Teddy. I ended up leaving a job I had hated for years: after Teddy died I just couldn't do it any more and handed in my notice. Afterwards, I realised it was one of the best decisions I ever made: it is such a waste of life to spend it doing something you don't enjoy. Someone I talked to about it later on suggested that perhaps that was Teddy's gift to me. I don't know, but I do know that since Teddy died I make sure that I make the most of every single day I am given with my beloved dogs. Life and love are both very precious.
As for how to cope, there aren't any easy answers but take it one step at a time, be kind to yourself and try not to have any expectations about how you "should" be feeling or when you "should" be feeling better .
Rest in peace, Tawny .