12 y/o cat suddenly passed. similar experiences? what happened?

i am haunted by “what-ifs” and guilt over suddenly losing my seemingly healthy 12 year old soul cat and compulsively scan reddit for similar experiences. based on these details, can anyone please devise what likely happened or if they had a similar experience? was it heart attack? HCM? early kidney failure? how do you stop the what-ifs and move forward in grief?

long explanation below:

it’s been 15 days since my dad found my seemingly healthy soul cat passed away on his side with his eyes open in the basement at around 8 AM. my dad thinks it must have happened sometime around 3-5 AM, as they were awake rushing my mom to work at 6 AM and my cat didn’t come out (everyone just figured he was asleep or hanging out in the basement, where he liked to sleep on pillows and blankets, and where his cat litter and dry food were). my parents, who sleep on the floor above the basement, didn’t wake to any sounds. my cat was last seen loafing next to my dad on the couch at 11 PM for an hour or so, with no signs of discomfort/anything irregular. before then, from 7-8ish, he jumped up on my chair with me at dinner like he always does, loafing beside me. i’m so deep into it that sometimes i think my petting his head in a different way caused a stroke. just days before, i have a video of him on his cat tower beside our TV, energetically swatting at Olympic skiers. he was his same playful and affectionate self.

we didn’t get an autopsy because we didn’t want to disturb his body and there was no way to go back and save him. but in hindsight we’ve been combing over every tiny thing looking for signs that we missed.

we had him on a mix of friskies gravy wet food cans which he happily ate for years, as well as a mix of senior kidney-supportive wet food that he gobbled up every morsel of. (this we had for our 19 y/o cat who we let go last may when his renal/kidney failure got bad. we were used to that not being the 12 y/o’s food.) this cat looooved his wet food, which he begged for on his routine of breakfast and dinner. but favored the senior cat food, so we didn’t think too much of him only licking the gravy of the fancy feast when he was still eagerly eating the senior food. we even ordered him new senior fancy feast that arrived the day he passed away, which was gutwrenching. but was this a sign of lack of appetite?

my dad also noticed for the past two days prior that he was drinking more water than usual. but we didn’t think anything of it because he always loved drinking lots of water, ever since he was little. he had a funny habit of putting a paw into the bowl to hold it steady or using his paw as a spoon. he was so funny. but was increased thirst a sign too?

my dad monitored his excretions due to our experience with kidney-failure and saw no struggles/anything irregular.

a week prior i had looked into why he was a little clingier than normal, and wanted to take him to the vet, for the fear that it was a sign something was wrong. thinking it wasn’t urgent since that was the only oddity we flagged, we never got around to it. it was especially easy to overlook because he was an extremely loyal cat who loved quality time and snuggles to begin with. he was just even more so cuddlier and clingy, climbing onto my chest multiple times over the past few months and sitting on everyone’s laps fairly often.

we all know that, regardless of his signs, he should’ve been screened at the vet in general since he was 12, going on 13 in may. that will haunt me for the rest of my life, and i apologize to him every day. but what happened to my sweet baby? why was he on the basement floor? did he quickly collapse and go with minimal, hopefully no pain? i’m so heartbroken and feel like an absolute failure, even though i know we gave him great care and so much love.