When it comes to what Paul is saying here about not losing heart, I imagine that it is more about seeing ourselves as a balloon with a slow leak in it. We feel deflated. We lack spirit and purpose. I think we all like those helium balloons that are lighter than air and go up and not down. Yet, even they come down somewhere. Even without an obvious leak the helium escapes and the balloon sinks.
So, how does Paul not lose heart? How come Paul is not a quitter when he must feel like washing his hands of them and moving on to those who might give him a better reception?