How do I fix this cat?

Sorry to hear. Peace and comfort to your family.

I've had to put down two canines. Vet was awesome, situation was about as comfortable as it could be made, but it was still a tear-filled process. Older dog was a 13-year old lab mix, and his rear hips just gave out. Younger was a beagle/rat terrier (10-11 years old) mix (yeah, weird, I know) and she had some sort of stroke; kept walking in circles and stopped eating and drinking. We just knew and the Vet confirmed.

Make every effort to recount and enjoy the memories of their life; rather than the episode of finality.

It's just something you have to prepare yourself for when deciding to be a pet owner.