Saint Theophan the Recluse wrote to a struggler:
You write that you prayed fervently and at once you were calmed, receiving an inner assurance that you would be released from oppression; and then, indeed, it was so…
Recall how you prayed and always strive to pray this way, so that prayer comes from the heart and is not just thought by the mind and chattered by the tongue.
I won’t conceal the fact that, though once you prayed from the heart, it is hardly possible to pray that way constantly. Such prayer is given by God or is inspired by your Guardian Angel. It comes and goes. It does not follow, though, that we should give up the labor of prayer. Prayer of the heart comes when one makes an effort; to those who do not strive, it will not come. We see that the Holy Fathers made extraordinary efforts in prayer, and by their struggles they kindled the warm spirit of prayer. How they came to this prayerful state is illustrated in the writings they have left us. Everything they say about striving in prayer makes up the science of prayer, which is the science of sciences. The time will come when we will study this art. But now, since it came up in our correspondence, I touch on it only in passing. Let me add: There is nothing more important than prayer; therefore, our greatest attention and most diligent attention must attend it. Grant us, O Lord, zeal for such an effort! — St. Theophan the Recluse (Letter 15)