St. Paul writes in 2 Corinthians 12:7-12:9:
And lest I should be exalted above measure by the abundance of the revelations, a thorn in the flesh was given to me, a messenger of Satan to buffet me, lest I be exalted above measure. Concerning this thing I pleaded with the Lord three times that it might depart from me. And He said to me, “My grace is sufficient for you, for My strength is made perfect in weakness.” Therefore most gladly I will rather boast in my infirmities, that the power of Christ may rest upon me. Therefore I take pleasure in infirmities, in reproaches, in needs, in persecutions, in distresses, for Christ’s sake. For when I am weak, then I am strong.
St. Silouan the Athonite (d. 1938AD) writes about his own experience with a thorn in the flesh:
“The love of God gives strength to continue in prayer the whole night through, but pain in my head wears me out and I am obliged to give up and rest. These headaches have been given me because I insisted on having my own way, and left my task as steward to go into the ‘desert’ to have great freedom for prayer; but the Lord wanted me to spend my life in the Monastery as steward. Twice they would have made me prior, and once senior steward, but each time I refused, and for that God punished me. It was only later that I understood that everyone is needed in the place where he is, and we may all be saved whatever our office.” (St. Silouan the Athonite, pgs. 465-466)