By 'over many lifetimes' it's meant that, literally, Zero has done this song and dance in her world over many separate and individual lifetimes. Not in the form of rebirth but in the form of repeating time cycles. Each time Kaguya fails to bring back Fiethsing and almost destroys the world instead, Alhama'at goes back and attempts to kill her which results in the very problem that leads to Kaguya attempting to fix things. Zero is one of the few characters aware of this cycle and it's part of why she's as hopeless and depressed as she is typically. She isn't consciously aware of all the details but like Gill she suffers deja vu a lot. Zero has never once seen a good ending. Of course she'd say that this world seems constantly beset by a cycle of misery.