It is worth remembering where this famous verse sits. Lamentations is a book of grief over a ruined city, and the chapter it belongs to has just described a man who says God has broken my bones and made my chain heavy. This is not written from a comfortable place. It is written by someone with every reason to conclude that God has finished with him.
Then comes a hinge: This I recall to my mind, therefore have I hope. He is not reporting a feeling. He is deliberately calling something back — the steady fact that God's mercies are new every morning and his faithfulness is great. Hope here is an act of memory, not a mood.
That is a practical model for a hard season. There will be mornings when nothing in your circumstances has improved and nothing in your emotions cooperates. On those mornings you are not required to manufacture optimism. You are invited to recall to mind something that is true whether or not it feels true, and to let the recalling do its slow work.
New every morning also means yesterday's supply was never meant to cover today. Whatever you spent getting through yesterday, it is not your reserve. The mercy arrives fresh, on the day, in the amount the day requires.