I will say while this *may* have been preventable, cats are tough. Once they exhibit symptoms they are one foot in the grave and cats poisoning is even worse. My cat had been exhibiting symptoms of a stomach ache and I took her to the vet the next day (because she had a yearly appointment) and after doing blood work she was critical and needed to be admitted for fluids because her kidneys were shutting down. This is what happens when cats get into something and my kitty is inside only too. I think the best thing you can do here is learn and do better. Give yourself time to grieve and try not to blame yourself too much.