I forgot a lot of what I had studied, so I was pretty disappointed.
この場合、次のような言い方ができますよ。
ーI forgot a lot of what I had studied, so I was pretty disappointed.
「勉強したことをかなり忘れていてがっかりした」
to forget what I had studied で「勉強したことを忘れる」
I was pretty disappointed で「かなりがっかりした」
ーI was reviewing some English I had studied, and didn't remember a lot of it. I was pretty bummed out.
「勉強した英語を復習していたら、かなり覚えていなくて、がっかりした」
to review で「復習する」
I was pretty bummed out で「かなりがっかりする」
ご参考まで！
I reviewed my English after a long time, but I was sad to find out that I had forgotten quite a lot of what I had studied earlier.
ご質問ありがとうございます。
"I reviewed my English after a long time,"＝「久しぶりに英語を復習しました、」
"but I was sad to find out that"＝「でも～だと分かり悲しくなりました」
"I had forgotten quite a lot of what I had studied earlier."＝「以前、勉強したことをかなり忘れたということを。」
ご参考までに。
カナダ 
アメリカ合衆国 
