From A Godward Life, John Piper:

Do not be anxious about tomorrow, for tomorrow will be anxious for itself. Sufficient for the day is its own trouble. (Matthew 6:34, Piper's translation) 

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With each day's measure of pain, [God] gives new mercies. This is the point of Lamentations 3:22-23, "The Lord's lovingkindnesses indeed never cease, for His compassions never fail. They are new every morning; great is Your faithfulness." 

God's mercies are new every morning because each day has enough mercy in it only for that day. That is why we tend to despair when we think that we may have to bear tomorrow's load on today's resources. God wants us to know that we won't. Today's mercies are for today's troubles. Tomorrow's mercies are for tomorrow's troubles. 

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The manna in the wilderness was given one day at a time. There was no storing up. That is the way we must depend on God's mercy. You do not receive today the strength to bear tomorrow's burdens. You are given mercies today for today's troubles. Tomorrow the mercies will be new.