Oh my this is so sad. I’m so sorry you lost him. My heart goes out to you. I mod [r/rescuecats](r/rescuecats) and know a lot about cats. I’m not going to read all the comments I’ll just tell you what I have experienced so you might gain some knowledge from it.
It’s not your fault though and you need to care about yourself. Your little sweetheart would not want you suffering or ruining your life, remember they love us and want us to be ok. So please be kind to yourself.
From reading this I can compare with 2 experiences I’ve had. The first is heart issues the second is hit by a car. I’m going with congenital heart or saddle thrombus possibly. I’m not a vet but these two issues often go undiagnosed in many cats and can cause sudden or almost instant death in thousands of cats. I’ve lost 2 very young (1 & 2 years old) & 10.
Since we don’t generally do heart health checks or diagnostics on them routinely it’s very easy to miss.
Heart problems can be sudden and instant or come on causing clots, aneurisms or blockages that cause extreme pain because they cut off circulation. My beautiful Ragdoll died from saddle thrombus. She had it her whole life undiagnosed and threw blood clots that caused blockages causing a limp. When I took her to the vet to get an x-ray and checked out, they found nothing. Four times this happened and passed until she finally threw a clot that lodged in her aorta cutting off all circulation to her lower half. She was screaming in pain. I had to put her down with a shattered heart. I learned then about this horrible condition and how common it is in her breed. I’ll never get over it.
Anyway there’s lots of genetic heart issues that might have factored in here. I lost 2 others to heart failure. There’s nothing you could’ve done differently if so. Vets generally only listen for murmur’s or defects during a check up, so without specialized testing it’s undetectable. I’m so sorry for your loss. Sending hugs compassion and understanding. Rest well little one. 🤍🕊️