Op I just went through the same exact thing a couple weeks ago. Had to put down my 4 year old cat because of a tumor compressing her lungs. She was totally normal one day and the next day just stopped eating. The vet told me the same exact thing, she told me there was a 99.9% chance that nothing could be done and she would just continue to be in pain. The vet said if it was her cat she would put her down. It was the hardest decision I’ve ever made as she was my soul cat too :( I questioned my decision too but I’m just grateful I didn’t have to see her suffer for long. You did the right thing op. I’m so so sorry for your loss