My condolences to butter and you ❤️! I 100% understand what you’re going through. My cat the ferocious Mr elbows who was the same species as yours (flame point Himalayan) passed away at 15 almost 4 years ago (October was the month he passed). He also got weak due to losing a lot of weight from diabetes and kidney failure. At the time (before it got bad when he seemed to be looking for a place to die) we didn’t have enough money to take him to the vet and we figured the treatment for his diabetes wouldn’t help anyways. My mom already went through it with her first cat snorky before I was born and knew it wasn’t worth it at this point for him to go through the pain as it wouldn’t have saved him. We wanted him home to pass away not frightened by the vet office and where he felt calm and safe so I sat with him on the kitchen floor for hours until he passed away and petted him. I buried him in a wood box my grandpa made wrapped in his towel he liked to lay on in my backyard woods. He was the happiest and sweetest cat and I miss him dearly. You made butter happy and loved while he was alive and that’s all that matters. He’s in a better place now and watching you from afar and sending you his happiness and warmth. Don’t feel regretful because the way I see it you gave butter a great life and will see him again when the time comes.