〜てしまう: completion and regret
Reading てしまう as a mistake every time can distort an intentional completion. Distinguish emphasis on finishing from an emotional response to an unexpected result.
Completing an action
この本はもう全部読んでしまいました。 says that the whole book has already been read. Together with 全部, it emphasizes completion; it does not necessarily mean the speaker regrets reading it.
今日中に片付けてしまいましょう。 proposes finishing the tidying today. The expression can refer to an intentional future completion, not only an unintended event in the past.
Reacting to an unintended result
財布を家に忘れてしまいました。 describes having left one's wallet at home. In this context, the expression readily conveys regret or inconvenience and can make the speaker's reaction more apparent than 忘れました alone.
The strength of the reaction depends on the vocabulary, intonation, and situation. てしまう alone does not establish whether the problem is serious or minor, or whether someone acted deliberately.
Connecting て-forms and contractions
食べてしまった may contract to 食べちゃった, and 読んでしまった to 読んじゃった. Recognize てしまう → ちゃう and でしまう → じゃう, while using a form such as 食べてしまいました when a polite explanation is appropriate.
Compare 読んでおく, reading in preparation, with 読んでしまう, reading to completion. One highlights preparation and the other completion; neither is simply another name for the past tense.
Check: Does 宿題を全部やってしまいました。 always express regret?
Show the answer and why
No. It can emphasize that all the homework is finished. Whether there is regret depends on the situation and the speaker's tone.