Not all words have antonyms, and there is no clear choice for the word memoir. Either you record your memories or you don't. The opposite of a memoir is not having a memoir. I might say that since memoirs are supposed to be factual records of events, the opposite would be a work of fiction, but that's stretching.
The opposite of diary is a shared or public record.
book ( already written )
life
It is spelled Dear Diary.
his diary
Diary - 2000 Diary of Ludacris was released on: USA: 2002
Kim's diary = the diary that belongs to Kim
Who keeps a Diary? The diary was full with secrets.
Diary - 2000 Best of Diary was released on: USA: 1 February 2001
Anne received the diary on her birthday. She wrote about receiving the diary and her birthday and the problems in Belgium and her life.
Her diary is called, "A Diary of a Young Girl".
The most famous diary is Anne Frank's diary.
The plural noun for diary is diaries.
A diary can be bought at a book store, but any book or notebook with blank pages can be a diary. You can also get an app for a diary on your device.
Diary - 2000 The Diary of Lindsay Lohan was released on: USA: March 2004