The darkest hour is just before the dawn
There is hope, even in the worst of circumstances.
This is one of those improving proverbs that are the stock in trade of the contemporary glut of self-help manuals and talking therapies. The darkest hour has long been used figuratively to mean 'the lowest ebb' and there are many such examples of it in print dating from the late 1700s.
The English theologian and historian Thomas Fuller appears to be the first person to commit the notion that 'the darkest hour is just before the dawn' to print. His religious travelogue A Pisgah-Sight Of Palestine And The Confines Thereof, 1650, contains this view:
It is always darkest just before the Day dawneth.
The source of the proverb isn't known. It may be Fuller himself, or he may have been recording a piece of folk wisdom. In 1858, much later than Fuller of course, Samuel Lover attributed the notion to the Irish, in Songs and Ballads:
There is a beautiful saying amongst the Irish peasantry to inspire hope under adverse circumstances:- "Remember," they say, "that the darkest hour of all. is the hour before day."