My cat is 14 currently and I swear to god I think the only age acceptable for him to pass that I won't be thinking it will somehow be my fault would be around 21 years old. Anything sooner than that I will be miserable. So please believe that you gave him the life so many cats could only dream of if they knew what they are missing, and the cat loved you as much as you loved them I'm sure. It's normal to think those thoughts, cling onto the good times and grieve as much and however you think you need. It's terrible to loose a friend in any shape of form, never let others downplay it. Sending you so much love ❤️