I am 99.9% certain that this story was a single post (SOLO), and that it is an older story

and this story is different as the MC is a diary

Mother finds her child's diary, after child's death, and reads last few entrys

mother adds last entry to the diary, while the diary is asking 'What are you doing?'

I do remember that there is an image in this story, it is of a yellow Post-It Note™®, although I can't remember what was written on it