We all know the feeling and feel so sorry for you.
We now have three cats buried in our garden - the two old ones who moved here with us, and a young stray that had become tame and was killed on the road. At least he too got the dignity of a burial in the garden.
I still chat to them when I pass that end of the garden even although we now have two one year old cats - very much loved too.
Sometimes I could swear I "see" on or other of the old cats, and even their predecessors - mind playing tricks, or just the fact that whether animals or humans we never really stop missing those we love.
You did the right thing, as you now, but it is cold comfort right now.