"Lose not heart"

Amazing words written by J.D. Salinger, whom I've never read,  and uttered by my brilliant friend Carissa who turns facts into storytelling knitting details into amazing tales that you never want to end.
Salinger wrote those letters to a fan and aspiring writer who had a piece rejected by the New Yorker.
But how can you lose not heart?
How can you simultaneously pour yourself into something and protect yourself from loss and disappointment? How can you at once produce and let go?
For all the complaining that is done about aging, I find it comforting that some things do get better with time. Sometimes, for instance, we don't need to answer questions right away.
Sometimes we can figure out temporary answers, impermanent answers, imperfect answers.  And they will have to do. Because it also becomes clear that the only way is up and ahead. That every setback is only that. Ultimately the cycles move and the tides change, and the only constant thing is permutation. It is wisely said in Spanish "más sabe el diablo por viejo que por diablo", the devil's knowledge comes from age, not from being the devil.
So tomorrow, when the wheels start spinning again, I won't lose heart, or impulse, or hope, or time.