From the Imitation of Christ, Book 3 Chapter 48:
The days of this life are short and evil, full of grief and distress. Here man is defiled by many sins, ensnared in many passions, enslaved by many fears, and burdened with many cares. He is distracted by many curiosities and entangled in many vanities, surrounded by many errors and worn by many labors, oppressed by temptations, weakened by pleasures, and tortured by want.
When, Lord, shall I think of You alone? When shall I fully rejoice in You? O good Jesus, when shall I stand to gaze upon You? When shall I contemplate the glory of Your kingdom? When will You be all in all to me? Oh, when shall I be with You in that kingdom of Yours, which You have prepared for Your beloved from all eternity?
I wish to cling fast to heavenly things, but temporal affairs and unmortified passions bear me down. I wish in mind to be above all things, but I am forced by the flesh to be unwillingly subject to them. Thus, I fight with myself, unhappy that I am, and am become a burden to myself, while my spirit seeks to rise upward and my flesh to sink downward. Oh, what inward suffering I undergo when I consider heavenly things; when I pray, a multitude of carnal thoughts rush upon me!
O my God, do not remove Yourself far from me, and depart not in anger from Your servant.
Pardon me also, and deal mercifully with me, as often as I think of anything besides You in prayer. For I confess truly that I am accustomed to be very much distracted. Very often I am not where bodily I stand or sit; rather, I am where my thoughts carry me. Where my thoughts are, there am I; and frequently my thoughts are where my love is. That which naturally delights, or is by habit pleasing, comes to me quickly.
Blessed is the man who for Your sake, O Lord, dismisses all creatures, does violence to nature, crucifies the desires of the flesh in fervor of spirit, so that with serene conscience he can offer You a pure prayer and, having excluded all earthly things inwardly and outwardly, becomes worthy to enter into the heavenly choirs.