Terri, stop flaggellating yourself over this. The fault that allows Sashimi to have this condition is in his genes and it may have been put there deliberately by a breeder, long ago.
I was very new to birds when I adopted my second tiel, Chip, froma Petsmart. Chip as a Normal Grey and he sang so nice. It was the reason I bought him; I was looking for a cock to pair with Gracie to keep her company while I slept during the day for work. I did not know Petsmart got their cockatiels from a breeder that bred cockatiels for medical experimentation. I later learned Chip was bred from a line of tiels which carried a gene for atherosclerosis. Gracie and Chip produced 23 chicks for me and I kept all but six. I was so surprised when Chip literally dropped dead just after his eleventh birthday. He was sitting on a wall perch beside Gracie, singing his heat out and suddenly stopped singing and dropped to the floor. I rushed over and he was dead. His tongue was bluish, so I figured it was a cardiac event. When I had him necropsied, he had extensive atherosclerosis in his heart and his brain. My vet said she betted he was a special genetic line developed for medical research. When I contacted the breeder indicated on his band, they admitted it was true. Losing Chip was terrible, but the worse part of the story is that every one of the cock chicks he fathered has also died between the age of 10 and 12 years of atherosclerosis.
I learned my lesson. Buy from private breeders, not from pet stores. Better chance of a long life. Sashimi's father could have been from one of those medically established lines.
Just focus on supporting his remaining time and treasuring his presence.