I first read this when I was 18, wrote it into the flyleaf of my Bible, memorised it, and often say it to myself when I am stressed.

   Teresa of Avila           (1515-1582)
 Lines Written in Her Breviary
Let nothing disturb thee,
Nothing affright thee
All things are passing;
God never changeth;
Patient endurance
Attaineth to all things;
Who God possesseth
In nothing is wanting;
God alone sufficeth.
                    —H. W. Longfellow (translator).