Lamentations 3 begins by recounting all of the sufferings the writer has gone through. Yet he has rediscovered hope: the Lord is good to those who wait for him. The tone of this verse in specific suggests a sort of internal struggle the author is going through to convince himself that his faith and hope in God are well placed.
This waiting upon the Lord is expectant, but it is filled with suffering and hardship. The writer is instructing himself and us as the reader about how we are to behave in our suffering. We accept suffering, we stop resisting, and we wait patiently, trusting that the Lord is ultimately good.
It is easy for me cognitively to affirm the goodness of God, especially when life is good. The hard part is to affirm the goodness of God when life is hard and filled with suffering. Sometimes we want to throw ourselves a pity party when life gets hard, and at other times we just get angry at God. But the writer here instructs us to intentionally seek God. The assertion that the Lord is good to those who wait for him doesn’t mean he is immediately going to make our situation better. But he does promise his comforting presence as we go through adverse circumstances.