"Insecurity" is built on repetition and growing numbers. Each pair of lines repeats itself almost exactly, and the count climbs from a hundred to a thousand, then a million, then a billion. The speaker seems to be measuring how heavy their self-doubt feels, and the numbers get bigger the more they think about it.
There is also a shift in the middle. In the first half, the flaws are ones "you show to me," so someone else is opening up. In the second half, the speaker turns it around: "I show them to you." The flaws are now the speaker's own, and the speaker is the one being vulnerable. The line "no one would assume" suggests these flaws are hidden behind a confident surface, and only one person gets to see them.
The mood is quiet, anxious, and a little tender. The repetition feels like a worried thought looping in the mind, but the trust in sharing it gives the poem some warmth.