Very sorry for your loss. Our cat died this January as well after 18 years and we knew she was leaving a month or so before she passed. "Oscar" (yes, Oscar was a she) stopped eating and although she didn't appear to be in pain, she progressively got weaker.
The day before she died she went downstairs to my daughter's room. I have no idea how she made it as she was only crawling at that point. I believe she went down there for the last time to say goodbye to that room as she had slept there quite often. I carried her back upstairs and later that night she died.
I believe that all creatures have a soul. Oscar is buried in the woods behind our home on a small hill facing a ravine. I placed her as if she was laying and looking out over that scene. I believe she is in heaven waiting for us, just as people are.
There are many areas with more information on the internet regarding pets and heaven. Try some of those and perhaps you will be able to accept and believe in something that will bring you comfort. Good luck.

Laughter and love,

Richinside