YOU SAID:
I was not smart for doing this.
INTO JAPANESE
私はこれをすることに頭が良くなかった。
BACK INTO ENGLISH
I was not smart about doing this.
INTO JAPANESE
私はこれをやることに頭がよくなかった。
BACK INTO ENGLISH
I was not smart about doing this.
That didn't even make that much sense in English.