A word to describe the state of forgetting something

Solution 1:

I will pick-up where tchrist left in the comment section. Actually, this word was my primary choice because you were looking for a word expressing a forgetful state of the mind.

OP if you dig down into the etymology of the word "Oblivion", the primary definition of oblivion (late 14c) is "state or fact of forgetting" from Old French Oblivion (13c). https://www.etymonline.com/word/oblivion#etymonline_v_2432

Also, there is always lexical semantics in the play, when we use the word in the given context the word may mean little different, but not far from the root meaning. Even here, the word mean has different meanings in the given context: 1. to intend and 2. unkind or unpleasant.

The secondary meaning of the word oblivion in the contexts is: the state of being completely destroyed or the state of being unconscious. I will also recommend you to check the Cambridge Dictionary on the other definitions of the word: Oblivion. https://dictionary.cambridge.org/dictionary/english/oblivion

The Cambridge dictionary defines oblivion as:

Oblivion (noun) :No Memory

Meaning: the state of being completely forgotten:

Usage: These toys will be around for a year or two, then fade into oblivion.

John realized he forgot something, and wondered how long was he in the oblivion.