I'm an animal lover as well and I can understand your feelings. When a pet or anyone we love suddenly dies, I think it's a very human reaction to search your heart and wonder if you could have done more or if we shouldn't have done such n' such. Here's what I have learned from this.
No one could have loved your dear cat more than you did. You know in your heart that you loved him and he knew you loved him. That's the most important thing. You loved each other and love doesn't die when this temporary shell dies. I know because I've experienced countless times having my loved ones, both human and pets, reveal to me in some way that they were all right. I then knew that I would see them again.
There is a verse in the Bible in the book of Ecclesiastes that says, "To everything there is a season..." and one of those is "a time to die". It was your beloved cats time and you will see him again some day.
There is a beautiful animation online called "The Rainbow Bridge"
I'm new here so I don't know if I'm allowed to post a link to it, but the website is called indigo org. Go there and scroll down to the bottom where the "search" thingy is and type in Rainbow bridge. I know it will comfort you.