しまった、ちゃった

The construction is usually after the ~te form of any verb

全部のお金を使ってしまいました。 zenbu no okane o tsukatte shimaimashita. Unfortunately, I spent all my money.

私は完全に日本語を忘れてしまった。watashi wa kanzen ni nihongo o wasurete shimatta. Unfortunately, I have completely forgotten Japanese.

Another very useful variation is ~chatta. This is informal and is used by both male and female speakers. chau is made by combining te shimau -> chau

試験に落ちちゃった。 shiken ni ochichatta. I flunked the test unfortunately.

or in the present tense

ケーキを全部食べちゃう。　ke-ki o zenbu tabechau. I will eat all the cake.

I learned both in the classroom, of course, since they are standard dialect. But when I moved here to Osaka, people started asking me if I was gay, since I kept using ちゃった all the time.

If I could help prevent someone else from having to go through the same "hard knocks" learning process that I did, it would make me very happy.