Dear Mary Willis,--The Collected Letters of C. S. Lewis, Vol. 3: Narnia, Cambridge, and Joy (Cambridge University Press, 2007), 962
(1) Remember what St John said 'If our heart condemn us, God is stronger than our heart.' The feeling of being, or not being, forgiven & loved, is not what matters. One must come down to brass tacks. If there is a particular sin on your conscience, repent & confess it. If there isn't, tell the despondent devil not to be silly. You can't help hearing his voice (the odious inner radio) but you must treat it merely like a buzzing in your ear or any other irrational nuisance.
(2) Remember the story in the Imitation, how the Christ on the crucifix suddenly spoke to the monk who was so anxious about his salvation and said 'If you knew that all was well, what wd. you, today, do, or stop doing?' When you have found the answer, do it or stop doing it. You see, one must always get back to the practical and definite. What the devil loves is that vague cloud of unspecified guilt feeling or unspecified virtue by which he lures us into despair or presumption. 'Details, please' is the answer.