I hate platitudes, but I did hear the saying something like grief is love with nowhere to go. The amount of suffering you feel is proportional to the amount of love you gave your cat, so it sounds like your cat was exceptionally lucky overall.

We lost our cat a few years ago. She was quirky and weird and sick her entire short life. She had lymphoma - the vet never even tested for it because she was too young. The day we were supposed to bring her home from an overnight stay was the day we had to put her down, and it scarred us deeply. My work sent me home, I was so useless. I cried for hours when I realised our other cat had stopped eating from half of the bowl they shared.

It sucks. It always sucks. It feels like it will never get better, and it won't, but it'll get dimmer. You'll be able to remember the good stuff more often without the final moments crashing in. It just takes time.

For now if you have to wallow in despair, do it. You lost a loved one. It's your right. Remember though, you're going through all this because your pet didn't have to. You held up your end of the deal, and your pet got a great life because you took the pain of loss away from them.