Discouragement is a reality in a fallen world. Lost men know discouragement regularly. But it's not just lost men who get discouraged. And it's not just unfaithful Christians who get discouraged. Discouragement is a reality even among godly people.
We have been brought, by God's grace, into the truth. We know the truth. We know we should cling to the truth. We know that our minds, and our affections, and our wills, should always be informed by the truth. But the Bible acknowledges the fact that even those who know the truth are sometimes overwhelmed by the difficulty of their circumstances. We are, after all, just dust. We meet with our own weakness in those season of discouragement.
Be encouraged. If you know Jesus, you haven't seen your best day yet, and when it arrives it is going to be an eternal one.