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.
It is spelled Dear Diary.
his diary
Kim's diary = the diary that belongs to Kim
Diary - 2000 Diary of Ludacris was released on: USA: 2002
Who keeps a Diary? The diary was full with secrets.
Diary - 2000 Best of Diary was released on: USA: 1 February 2001
The most famous diary is Anne Frank's diary.
Her diary is called, "A Diary of a Young Girl".
The plural of diary is diaries.
Anne received the diary on her birthday. She wrote about receiving the diary and her birthday and the problems in Belgium and her life.
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 Diary of Tony Hawk was released on: USA: March 2003