When we are in periods of darkness, our light and strength comes from remembering our experiences of God.
When I hear what you say about the workings of grace in yourself. I am filled with an almost tangible joy; that is how God in his goodness deals with his children when they display goodwill! The profound impressions that your soul thus receives are not given merely for your progress in these days of fervour when the dew of heaven falls in a tangible way for your consolation and encouragement, but also for times of trial and darkness, should the Lord condemn you to them, as he sometimes does even those he loves much. Then the memory of intimate communications of grace is a sustaining force: one lives, so to speak, on this nourishment.
Letter to Casimir Aubert, 10 November 1835 EO VIII n 551