Not that I speak because of lack, for I have learned in whatever state I am, to be content in it.
The first thing to notice is the verb. Paul says he learned contentment. It was not a temperament he was born with, and it did not arrive complete. It was a skill, acquired slowly, and he says as much twice in the sentences around it.
That should encourage anyone who assumes contentment is a personality trait they simply lack. If it can be learned, it can be practised, and practice is available to everyone.
The second thing to notice is that he writes this from prison. He is not describing a comfortable life he has arranged well. He is describing a settledness that survives circumstances changing, which is the only kind worth having.
So the goal is not to want nothing. It is to reach the place where what you have and what you lack stop deciding how you are.