Sorry to hear it. I know EXACTLY what you're talking about. Sigh. Sorry for your loss.
We have four cats here, all very young kittens except the old guy: Boodgie is amazingly 20 years old and he's had several health problems in recent years and we know he won't live forever but he's a one in a million cat. He's as much a part of our family as any human is.
We're amazed he's still with us frankly.
But the problem is - it's the way of things with these guys - they come into and out of our lives. When it's time, consider adopting another furry friend since they do need homes.