We all have good days and bad days. Sometimes projects may seem meaningless or impossible or for some other reason one might just hit a wall. The existential malaise hits all of us sometimes, I think: we seek meaning and don't find it. But there are some who construct meaning rather than simply seeking it, and in this conscious act of creation, we can find the space to move forward on the bad days.
In the end, our persistence is the most certain guarantee that things will become better. As long as we can strive for our own improvement, then we can grow.
These ideas are taken--somewhat loosely, perhaps--from Viktor Frankl's "Man's Search for Meaning."