noun
The noun form of 悔しい, expressing a mix of frustration, regret, and bitterness, often over one's own failure or an unfair situation.
試合に負けて悔しさでいっぱいだ。
I'm filled with frustration after losing the match.
彼は悔しさをこらえて、次の挑戦に備えた。
He swallowed his regret and prepared for the next challenge.