Lines 19-28 Summary

Get out the microscope, because we’re going through this poem line-by-line.

"Ain't got nobody in all this world,
Ain't got nobody but ma self.
I's gwine to quit ma frownin'
And put ma troubles on the shelf."

This is the musician is singing a verse of "The Weary Blues." It's a pretty typical blues song: the singer is lonely and there's no one to help him out.
But! He is going to stop feeling sorry for himself and move on.
The grammar and spelling are supposed to mimic the way a real bluesman would sing. This mimicry is called "dialect."
The standard English version of line 21 is "I am going to stop frowning," but who talks like that?
It's all about being genuine and showing the world that even "flaws" are beautiful.

Thump, thump, thump, went his foot on the floor.
He played a few chords then he sang some more–

The "[t]hump, thump, thump" is the piano player keeping the musical beat by stomping his foot on the ground.
His whole body is getting into the music.
Line 24 sums up one style of blues music, where the singer/musician plays some chords or notes on an instrument, stops playing, and sings a line of the song.

"I got the Weary Blues
And I can't be satisfied.
Got the Weary Blues
And can't be satisfied–

We're back to the musician singing, but now he's doing the chorus. The chorus can sum everything up, then makes a big statement at the end.
Sure, Lines 27-28 are a repetition of 25-26, but it's not exact.
Dropping the Is in the repeated lines lets the singer draw out some syllables. It adds some punch and lets the musician show his chops.