How to Pursue a Happy Ending in a World destined for destruction

The protagonist finds themself reborn into a world that is already doomed to destruction, essentially becoming a minor background character in this doomed world. The original world has already ended and humans who survived are living in a harsh, ruined environment.

On the day they awaken in this world, they make a firm decision:
even if this world is destined to fall apart, they will do everything possible to achieve a happy ending instead of simply following the tragic fate that awaits everyone else.

In other words, instead of accepting the world’s inevitable collapse, the main character decides to change their own fate and find happiness for themself — against all odds in a world on the brink of destruction.