Lost my best friend last night
You have my condolences. Sounds a lot like what my 16 year old cat, Stubby did this year. He just stopped eating. Kept losing weight. Vet couldn't find anything. You have your memories.
I read somewhere that it helps to still act like they are around. Calling them to come sit by you and such. I thought it was stupid, but I catch myself calling Stubby all the time. It makes me feel better.