Like much of the work of Walt Whitman, “Thumbprint” by Eve Merriam is an unabashed celebration of the individual and one's uniqueness. That quality is illustrated by the metaphor of the eponymous thumbprint, which, with its whorls, whirls, and wheels, is utterly unique to the individual. No two thumbprints are the same.
The speaker is clearly a strong-minded woman with the remarkable capacity, by no means rare among artists, to be the author of her own self, to take her innate uniqueness as an individual and recreate herself anew.
This is arguably what she's talking about when she says,
And out of my blood and my brain
I make my own sun and rain.
The speaker effectively creates her own “sun and rain,” her own inner states. She doesn't let her moods be dictated by events, as so many of us do. At any given moment, whether she's happy or sad is largely down to her and what she has or hasn't done. As we've already seen, she's an individual, and that applies to her emotional life as well.
In the last two lines of the poem, the speaker indicates that, whatever she'll become, it will be down to her. She will make an impression on the world, just like a thumbprint, that will be an expression of her unique individuality.