Feeling Guilty

Yesterday I had to euthanize my sweet cat, Loba, after 2 weeks of syringe feeding and various medicines. She was 11 years old. I feel responsible for her death. A few weeks ago I brought her in for a patch of fur missing on her leg, then later found she had fleas, which must have been brought in by the dog since she was strictly indoors. I treated her with stuff from the vet and then bought some carpet powder from the store that said it was safe for use around pets and children(which I have). A week later she became very ill and I took her to the vet where she received fluids, blood work and other stuff. The vet thought it was pancreatitis. A week later I opted for an ultrasound. Both he and the other vet believe she had pancreatic cancer, but couldn't be sure unless she had surgery, which he thought she might not survive. After I got home yesterday I was researching natural flea control products, and read that peppermint, cinnamon and other oils are toxic to cats. I can't stop crying feeling that I caused her death, I just feel so guilty. But if the product I used was to blame I want to be sure that they can't claim them to be safe for cats, which it says on their website and also approved by vets, they need warning labels. My husband thinks I'm crazy for thinking like this. She was just so special to me and really helped me when I was going through a difficult time in my life. I found her as a sick little kitten and when the vet told me her probable age, she was born the week my mom died, I knew she was meant for me. Taking care of her and then just having a kitten around at that time gave me something else to think about. She was so sweet and loved to be around me and sit on my lap to be petted. I just can't shake the feeling I caused her death.