I feel your pain. Our male Golden, Quinn was diagnosed with thyroid cancer in July with a prognosis of 2-3 months. In August, our female Morgan stopped eating. Four days later she was diagnosed with stomach cancer. It was so advanced that we had to make the hard decision right then and there. She must have been in pain for a while, but never showed it.
Meanwhile, Quinn is still going strong and shows no ill effects. He is constantly hungry and very active. Surprisingly, his thyroid levels are normal. Needless to say, we are spoiling him rotten and trying (and mostly succeeding) to enjoy every day he has left with us. You know when you take them into your life that that day will come, but it doesn't make it any easier.