The Cloud of Unknowing, penned by an anonymous English Christian mystic in the Middle Ages, says this:
This is what you are to do: Lift your heart up to the Lord, with a gentle stirring of love desiring him for his own sake and for his gifts. Center all your attention and desire on him and let this be the sole concern of your mind and heart. And so, diligently persevere until you feel joy in it. For in the beginning it is usual to feel nothing but a kind of darkness about your mind, or as it were, a cloud of unknowing. Try as you might, this darkness and this cloud will remain between you and your God. You will feel frustrated, for your mind will be unable to grasp him, and your heart will not relish the delight of his love. But learn to be at home in this darkness. Return to it as often as you can, letting your spirit cry out to him whom you love. For, if, in this life, you hope to feel and see God as he is in himself it must be within this darkness and this cloud. But if you strive to fix your love on him forgetting all else, which is the work of contemplation I have urged you to begin, I am confident that God in his goodness will bring you to a deep experience of himself.
I keep this prayer by Thich Nhat Hanh on my bed stand and sometimes say it aloud when I awaken in the morning:
Waking up this morning, I smile, Twenty-four brand new hours are before me. I vow to live fully in each moment and to look at all beings with the eyes of compassion.
When I am worrying a lot, I take a few minutes to slow down, breathe quietly, and silently repeat this prayer from the fourteenth-century English mystic Dame Julian of Norwich:
All will be well, And all will be well, And all manner of things Will be well.
This little portion of Psalm 19, from the Bible, is a constant prayer of mine:
Clear thou me from hidden faults.