Sorry to hear that. We had to do the same with my Mom's last cat Smokey (which we had 12 years) back in 2011. She had a large tumor and was almost 20 years old - not worth fighting with drugs esp since her kidneys were barely functioning. The hardest part was waiting after you knew it was time. The vet came to our house so Smokey could pass in familiar surroundings. Waiting for her to arrive seemed like hours, but it was only 20 min.
After our current cat William (going on 16) passes... we've decided no more. Get too attached, and I'm NOT getting any better at this.

Hang in there Yoyo. I know it's tough, but you're doing the best for them.
My sincere condolences.
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David (OFI)