My 12 year old boy left me yesterday

I rescued him as a kitten, only a few weeks old. He was abused in the few weeks of life. But after we stole him he was very spoiled. I was obsessed with him growing up and he was my favorite cat. A little bit after he turned 12 I took him to the vet because he was more clingy than normal and losing weight and not eating as much. Diagnosis was a two inch mass on his liver pressing on his little tummy. Bloodwork was normal but they did an ultrasound after. I was giving him medicine twice a day and doting on him even more than I usually do. I spent so much money buying him baby food, special treats from pet stores, giving him cans of tuna with extra water. He even gained some weight back. But the last few days he lost all the weight he lost and was not interested in any food at all. He would still come up and want to eat but he just couldn’t take any bites. I took him to the vet yesterday morning and did bloodwork and an ultrasound and the vet said she saw tumors everywhere she looked in his abdomen. I knew it was over and i took him back in before they closed that day to send him to heaven. The vet cried with me and said that he was the sweetest boy. He really was. Everyone who met him loved him, he liked to meow at people and would have conversations with you. He had the biggest green eyes and the softest fur. He liked to crawl under blankets with me and snuggle. He would follow me to the bathroom and sit by my feet waiting for me to play with him again or sit on my lap. He took my heart with him. My soul cat. I held him as he passed and wrapped his body in a blanket. I appreciated how the vet also cradled him like a baby while moving him to the back room. He was my research assistant too, he liked to get in my way when I was writing so I would have to write uncomfortably for hours so he would be comfortable on my lap. I miss him. I have so much guilt that I waited too long because he was obviously hungry but couldn't/ wouldn't eat. But he also would not try to eat? So I don't know. I don't deserve to eat while he was likely starving before I let him go. I failed my boy. I just hope he knew that I loved him so much and that he’s my child and that mommy will never forget him.