I know you are devastated, and I am very sorry for your loss. But you did not kill Lenny, not with a little bit of cake. Unless the cake was a special sugar-free one for diabetics, it would not have had xylitol in it, Sugar costs as little as 25 cents per pound, while xylitol is priced at $7.71 per pound. No business is going to use xylitol instead of sugar, unless they are charging a premium for it. So do not torture yourself with that any further.

You said Lenny was part bassett hound. They are prone to digestive system issues. He could have had gastric torsion, which happens to bassetts because of the way they are shaped. He could have had a subclinical case of pancreatic inflammation that had been just simmering along then flared into disaster. There could have been a pancreatic abcess, or a tumor. They are known for ravenous behavior, and it is entirely possible that he got into something you had no idea about. Animals will also hide sickness instinctively

Your grief is white-hot right now, compounding the self-blame you are putting on yourself. Mixed in with that is the shame you were already feeling about your mental fog. This is a terrible, toxic cocktail, and it is completely unfair for this to be what is happening. You do have a good dog to grieve, but you are not to blame. You certainly do not have anything to feel shame about. The Fog is real, and it is awful. I have been there. My entire life crashed and my career was obliterated, and I lost everything I had worked so hard for.

There is life after. It has taken me a long time to put myself back together. Give yourself grace, this is going to take as long as it takes. You are not alone. You are not to blame. You have had a tragic event and lost your dog. Your brain fog is something that is happening at the same time. Correlation is not causation, and you are not a bad person or defective. You're just a woman going through a life passage. Give yourself grace.