YOU SAID:

It was at that time that I realized I made a huge mistake

INTO JAPANESE

その時私は大きなミスを犯したことに気付いた

BACK INTO ENGLISH

At that time I noticed that I made a big mistake

INTO JAPANESE

その時私は大きなミスをしたことに気づいた

BACK INTO ENGLISH

At that time I noticed that I made a big mistake