St. Joseph (Litovkin) of Optina: That which is imposed is difficult

A woman said to [Elder Joseph], "Give me a rule of prayer; but I fear I'll find it difficult." The Elder answered, "That which is imposed is always difficult." Thereupon he told about Archimandrite Isaaky, who while living world was preparing for monasticism by ascetic feats; he did a thousand prostrations daily. After he entered the monastery, he told the Elder Lev about this and he had him do fifty prostrations instead of a thousand. In a short time he came to the Elder and said, "Forgive me, Batiushka, I'm ashamed to confess it, but for some reason I can't do fifty prostrations." The Elder had him do twenty-five. A little time passed and he again appeared before the Elder and said that could in no way understand why when he was living in the world he had been doing a thousand prostrations, whereas here he couldn't do even twenty-five. Then the Elder Lev explained to him: "In the world the enemy was helping you. You did them and became proud of it; but here you are doing them not of your own will, but from obedience. Now you can recognize your feebleness and are being humbled; that is why it is difficult."
+St. Joseph (Litovkin) of Optina, from Elder Joseph of Optina, Chapter V, The Elder Joseph's Instructions.