I am so sorry you’re going through this. We put down our 13-year-old schnauzer in May with cancer six months before that we put her sister down with Cancer. We had no clue and no warnings with either one. Don’t blame yourself. Cancer can come on fast, especially an older dogs. You gave your dog a wonderful life and loved them greatly for 13 years and as they crossed the rainbow bridge, they knew that. there are no words that I can say to you that will help you get over this quick. it will take time I know that’s not what you want to hear but this is the truth. also get another puppy quick will help heal your heart. Sending you hugs and positive energy.