You are torturing yourself and you need to stop or else you'll ruin the rest of your life. It's easy to second guess the vets/your mother and everyone else but take a deep breath and step back ... what if the problem with the large wound was a sign that your kitty was already far gone, rather...
I feel your pain MnM - it's always tough. And while you are distressed about your kitty dying in a vet's surgery, just remember you did the right thing for her.