The poem starts with an idea that doubles back on itself: the biggest fears have no denouement, no resolution, and that lack of an ending is a fear in its own right. The speaker admits that when people ask, they say roller coasters scare them. It is a safe, easy answer, and the poem quietly undercuts it with a short line: "But they end." A roller coaster frightens you, then lets you off.
The second stanza turns to fears that don't let go. They leave us trembling, legs giving way, tears that start and stop. The questions "Until what / Until when" feel stranded, as if the speaker is waiting for relief and can't find out what would bring it or how long it will take.
The last two lines are blunt. For the worst fears there is no closure, and the speaker calls that "earth shattering." The mood is uneasy and raw, held together by short lines and a sense of being stuck in the middle of something.