Oh I am SO sorry; my cat had cancer too and finally, he was in so much pain, we had to have him put down too.
It rips your heart out to put them down. My cat was starving to death. Would not eat anything. Would pace to her bowl, turn around and drink water and back to the bowl. We tried every type of food imaginable. Spent a fortune on cat foods and even tried fresh fish or cooked chicken or no salt added tuna, etc. Would not eat. Tried AD, ID, and so on and so on. Back and forth to the vet for tests. Nothing. She was losing weight rapidly and staggering weakly. It was heartbreaking. Noon last Saturday she passed in my arms. Her ashes are next to me. They will be spread on her favorite nibble on plant in the spring.