Tafsir of Al-Hashr 59:19

(And do not be like those who forgot Allah), meaning they forgot His rights—exalted be His Majesty—and did not estimate Allah with the estimation due to Him, nor did they observe the requirements of His commands—glorified be He—and His prohibitions—mighty and majestic be He—with the observance due to them.

(So He made them forget themselves), meaning Allah—the Exalted—due to that, caused them to forget their own souls, such that they did not strive for what would benefit them, nor did they perform what would save them. Or, it means that He—majestic is His Majesty—showed them on the Day of Resurrection such terrors that caused them to forget themselves; that is, He showed them a terrifying matter and a painful punishment.

As for the "forgetting of the self" in reality, it is said: it is something that cannot occur, because knowledge of the self is immediate (presentational). In this [assertion], however, there is consideration [for debate]. And [it is read as] "wa-la yakunu" (and they should not be) with the letter ya for the third person, by way of shift (iltifat). Ibn Atiyyah said it is a metonymy for the "self" (nafs), intended to denote the genus.