て-Form + しまう as “completely VERBed” (with an emphasis on finality)
The phrasing しまう is added after the て-Form of a verb to emphasize that the verb has completely occurred. This expression is almost entirely used to indicate a sense of regret because of the irrevocable resultant state that the completion of the verb has caused.
Kinou watashi no inu ga shinde shimaimashita.
(Yesterday my dog died.)
Sumimasen. Ukkari, uchi ni shukudai o wasureteshimaimashita.
(I’m sorry, I carelessly forgot my homework at home.)
Benkyou shinaide neteshimatta.
(I fell asleep without studying.)