Worse is when you fight when you shouldn't have.
Our four year old pound rescue, a lab/pitt/rott mix Eleanor got auto-immune hemolytic anemia, and was actually sick for a week or more, but we couldn't tell until she slowed down and went off her food the second to last day when her blood count was getting so low she was anemic, and her kidneys were starting to fail from trying to expel all the hemoglobin. We had no clue until the morning we caught her urinating blood.
Took her to the vet, she was slow, but still walking under her own power, and still perking up to wag her tail to greet people etc. The vet gave me 60/40 odds, and sent me to Walgreen's to get some pills that were for human organ transplant rejection immune suppression to try and stop the blood cell destruction.
We couldn't afford a 24/7 emergency vet so she spent the last 12 hours of her life alone the vet's kennel, where her IV line kinked and she wasn't getting hydrated, and we had to put her down the next morning when the staff came in the next day, neurological damage from the massive buildup of dead blood cells and hemoglobin had set in, and she could no longer move, but she still twitched and jerked when she saw my wife and I come in to see her.
It cost me $1200 to make her suffer alone for an extra 12 hours. In hindsight I'd have paid $10,000 to have her put down the day before right then and there.
At least I won't make that mistake again.