My Pebbles died

One of my barn kittens died. I feel horrible. I wormed her last week and I noticed afterwards she got really really skinny. I'd been feeding her canned food as often as she would eat it and she seemed to be perking up the last day or two. I had an appt for her for friday. Last night we noticed she had the runs pretty bad. I figured/hoped she would make it until friday. I found her laying outside the garage today when I got home from work. I feel so bad. I feel like its my fault. That if I would have gotten her in sooner. Or noticed sooner? I don't know. Things die I know...thats how it goes but I always feel so bad like there is something I could have or should have done. Why do I always feel this way?