Your little sweetie will always be w/you. That's quite a long life for a kitty, as my RB Pidgelet would tell you; she lived to be almost 22 and I'm sure she was there to greet your Tabby. Believe me when I say that your vet is right, that there was nothing done or not done that hastened her passing. Our bodies just simply wear out and our furkids are no different. She is now at peace and I pray that you will find some in your heart, too.