忘れてしまう

end up forgetting; forget completely

→ 忘れる
phrase Rare #40,149
end up forgetting; forget completely しまう can emphasize that the forgetting is complete or regrettable.
JA: 俺のことなんて、いつか忘れてしまうさ。
EN: You’ll forget about me someday.

最も大切な神の存在を忘れてしまう

I end up forgetting about the most important thing of all: the existence of God.

— 道 (1954)

俺のことなんて、いつか忘れてしまうさ。

You'll forget all about me someday.

自分の見た夢を思い出せる人もいれば忘れてしまう人もいる。

Some people can remember the dreams they had, while others forget them.

でも途中でワインを飲んですべてを忘れてしまうの彼が帰宅したとき

But he drinks wine along the way and forgets everything by the time he gets home.

— 甘い生活 (1960)