In-anime credits:
わたしのきもち
Performed by: Ai Hayasaka (早坂 愛)
Lyrics: Saka Akasaka (Idono Naka)
Music: Giorgio Giorgio
Actual credits:
悲しくて lulululu
From: オリジナル・サウンドトラック vol.1
Performed by: Yumiri Hanamori (花守ゆみり)
Composed by: Keiki Nishida (西田 圭稀)
Arranged by: Koki Adaniya (安谷屋 光生)
Lyrics by: Aka Akasaka (赤坂 アカ), Kiyomi Kumano (くまの きよみ)
From Kaguya-sama: Love is War Season 3 Ultra Romantic (かぐや様は告らせたい ～天才たちの恋愛頭脳戦～)
Certified best girl.
More Ai Hayasaka: https://youtu.be/vRKJvMQwgqU
Note: There are two full versions released. This is the version by Smithee Herthaka, the alter ego of Ai Hayasaka.
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