YOU SAID:

I'd forgotten you've mentioned that already.

INTO JAPANESE

私はあなたがすでにそれを言及したことを忘れていた。

BACK INTO ENGLISH

I forgot that you have already mentioned it.

INTO JAPANESE

私はあなたがすでにそれを言及したことを忘れていました。

BACK INTO ENGLISH

I forgot that you have already mentioned it.