What a Terrible Day :(

I noticed the past couple of days that Zeus has been really lethargic. I called the vet and asked if I should bring him in. They said to give him some wet food with his dry food and see if he eats more and keep watch of him. So I bought wet food and sure enough he was eating it and drinking water. But he we still just soooo lethargic and walking funny. He was hardly meowing and just seemed like depressed or something. So this morning, I found him laying against the wall in a weird position and I thought, that doesn't look right. So I picked him up and he started having a seizure. :(
I called my mom and she came over to watch the babies and I took off to the vet, no makeup, hair not done, nothing. I barely remembered to change out of my pajamas.
The doctor said he was critical and that he's become hypoglycemic. Basically he was freezing. His body temp was 94 degrees (should be close to 100), and his glucose level wouldn't even show on their machine it was so low. They had him on hot water bottles and were giving him sugar water. He was starting to come around and then just like that, he was gone.
I feel horrible. This is the second cat I've lost in a year. I only had Zeus two weeks, but I grow attached to animals really fast.
The doctor said that he was not 2 months old like the breeder said, but that he was closer to 5 weeks and had possibly not been weaned from his mother yet. :( But she said even so, he should've been fine eating kitten chow. She thinks there was definitely more of a problem, possibly with his liver or the way he processes sugar.
I'm so sad. He was such a sweet kitty and I wish he didn't have to go.
Ugh, I just can't believe it!
And to top it all off, come to find out the breeder we got him from is not registered. So he's basically breeding illegally. We're going to see if we can report him to animal humane.
Sorry to be such a debbie downer. :(