I might be thinking ~ inside, but I wouldn't actually say that out loud.
What I think and what I say are two different things.
ご質問ありがとうございます。
① "I might be thinking ~ inside,"＝「内心、～と思っているかもしれない」
"but I wouldn't actually say that out loud."＝「でも、それは実際に声には出さない。」
② "What I think and what I say are two different things."＝「思うこと、そして言うことは異なることです。」
ご参考に。
I thought in my heart I didn't want to lend him the money, but I didn't tell him that.
この場合、次のような言い方ができますよ。
ーI thought in my heart I didn't want to lend him the money, but I didn't tell him that.
「彼にお金を貸したくないと心の中では思っていたが、彼にはそのように言わなかった」
to think in my heart .... で「…と心の中で思う」
ご参考まで！
アメリカ合衆国 
カナダ 
