The speaker looks back on something frightening from years ago, which they describe as a nightmare or a scary story. If anyone asks, they say they don't remember, but the poem quickly admits the truth: they do. That small confession, repeated in "I do. / I do," is where the poem turns from hiding to standing firm.
From there the speaker takes the role of defender, protecting "the others who would otherwise fall" victim to the monster. The monster is never named or explained, so it could be a literal person or a symbol for something harmful the speaker survived. The poem leaves that open, which makes it easy to read in more than one way.
The mood shifts from uneasy and guarded to defiant. The final lines are short and steady: "I am smarter, / I am braver, / You will not win." The speaker doesn't claim to be bigger or stronger, only that courage and wit will be enough.