...a thorn was given me in the flesh, a messenger of Satan to harass me, to keep me from becoming conceited. Three times I pleaded with the Lord about this, that it should leave me. But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly of my weaknesses, so that the power of Christ may rest upon me. (2 Corinthians 12:7b-9, ESV)
"God’s grace is illustrated and magnified in the poverty and trials of believers. Saints bear up under every discouragement, believing that all things work together for their good, and that out of apparent evils a real blessing shall ultimately spring—that their God will either work a deliverance for them speedily, or most assuredly support them in the trouble, as long as he is pleased to keep them in it. This patience of the saints proves the power of divine grace.... He who would glorify his God must set his account upon meeting with many trials. No man can be illustrious before the Lord unless his conflicts be many. If then, yours be a much-tried path, rejoice in it, because you will the better show forth the all-sufficient grace of God. As for his failing you, never dream of it—hate the thought. The God who has been sufficient until now, should be trusted to the end" (Spurgeon, Morning and Evening, Morning of March 4).
See also: The Gift of Thorns