St. John of New Valamo: The mind should not be idle.

Imagination and memory are one inner sense. Sometimes the memory of former events [i.e. sinful acts] hits us on the head like a hammer. At such a time concentrated prayer is needed, and patience too. Our memory must be filled by reading the Holy Gospel and the writings of the Holy Fathers; in other words, the mind should not be idle. Former events must be replaced by other thoughts, and gradually the former recollections will be crowded out and the melancholy will pass. In one heart two masters cannot live together.
+St. John (Alekseyev) of New Valamo, from Christ is in Our Midst: Letters from a Russian Monk, letter #26, dated Dec 20, 1947.