I feel selfish and terrible, and yet can't give up. Am I wrong?

Three days ago I took my Cat to the vet because he was throwing up and losing a lot of weight. Two days later, I’ve racked up almost $5000 in vet bills for Feline Lipatic Stenosis. (fatty liver disease). On one hand, I feel I have to save my lil’ buddy. On the other, I feel can’t afford to do it. I feel like such the Judas goat. How will I know to say enough? My heart is trying hard to break! I need help!