This sentence in bold is from a text in which someone tells about how the death of their mother has affected themselves even after years, although she tried many ways to cope with it. Here is the paragraph with the sentence and the link
"On my way home after that workshop session, various difficult memories I hadn’t thought about for a long time suddenly resurfaced, as if my brain had been given permission to open the floodgates. I cried hard that night. But since then I have stopped being haunted by my mum’s final moments. Not that grief ever ends. *You learn to exist with it. There is a stone in your heart. Sometimes, it is big; it rubs and makes you bleed. Sometimes, it is small and you can almost forget it is there.
The structure of the sentence "Not that grief ever ends" is different. It is not in a usual structure, so I couldn't quite be sure about the meaning. I thought that:
It may be an inverted version of a standard sentence structure: (subject+verb+object) "That grief does not ever end."
It may also be a shortened version of the stucture commonly used such as "It is not that I don't like cooking, I just don't have time."
Similary, this sentence may be a shortened version of "(It is) not that grief ever ends, ....(meaning, Don't imagine a situation where this grief will someday go away, there is no such thing it going away, it would be a wrong assumption if you consider it that way, you just have to learn to exist with it.)
So, which one is it? Is it a shortened form of "It is not that grief ever ends...."? OR is it an inverted structure of a regular sentence which would have been "That grief does not ever end."?