"ケセラセラ" is Mrs. GREEN APPLE turning a shrug into a battle cry. The title is the Japanese spelling of que sera, sera, "whatever will be, will be", and the singer chants it like a daily spell: "今日も唱える" [I chant it again today]. It's less a lazy "who cares" and more a way to get through a bad day without falling apart.
The opening is pure stubbornness: "限界?上等 やってやろうか" [My limit? Bring it on, let's do this]. 上等 here is a tough-talk word meaning roughly "fine by me, come at me". Then comes a great line: "愛を捨てるほど暇じゃない" [I'm not idle enough to throw love away]. She's too busy surviving to give up on caring.
What I love is how small the comforts are. Painkillers don't fix "胸のズキズキ" [the throbbing ache in my chest] (ズキズキ is the sound-word for a pulsing pain), but a stranger's tiny kindness eases it a little. And on hard days she allows herself "ちょっとだけご褒美を" [just a little reward]. Very relatable.
The heart of the song is self-acceptance with a wink: "私を愛せるのは私だけ" [The only one who can love me is me]. Asked what she'd be if she were reborn, she answers "「また私だね」" [Me again, I guess]. She even admits she can be "自分よがり" [self-centred], which is refreshingly blunt.
There are some fun video-game touches. "はじめから" [from the beginning] and "つづきから" [continue] are the classic options on a Japanese game's start screen. Early on she wishes she could start over; by the end it's "ファンファーレ 明日も「つづきから」" [Fanfare, tomorrow I'll hit "continue" too]. ツァラトゥストラ is Zarathustra, the prophet-figure from Nietzsche's book, dropped in as a grand, slightly tongue-in-cheek nod. And the warning "ベイベー 大人になんかなるもんじゃないぞ" [Baby, being a grown-up isn't something you should do] is half joke, half sigh.
It ends right back at the title, spelled out in Japanese: "なるようになるのさ" [What will be, will be]. Not giving up, just refusing to stop: "勝てなくたっていい 負けない強さを持ちたい" [It's fine if I can't win, I want the strength not to lose].