〜てしまう

The te-form of a verb + しまう has two sences, which at first might appear rather inconruous. One sense it means carries the verb out with determination.

ほんをよんでしまいました。

I read the book comletely./ I finished reading the book.

The second sense is lack of control on how things turn out. This is usually pared with a sense of regret, something happens regretably.

でんしゃのなかにかべんをわすれてしまいました。

I inadvertently left my bag on the train.