The context of this verse has to do with Paul's relating his personal experience of God's sufficient grace in his life. He speaks of a particular malady, unknown to us, (perhaps so we will not be tempted to compare our miseries with his and reason ours to be worse). This ailment he refers to as a "thorn in the flesh,a messenger of satan", the purpose of which was to keep him from becoming proud. God's answer to Paul's thrice prayed plea for its removal was, "My grace is sufficient for you..."
"Sufficient" in our understanding means "enough", but in the Greek, the definition is a bit more revealing. It carries the idea of "raising a barrier, so as to ward off"; it is also translated as "be content". Think of it! In our trials, maladies, etc. God raises a barrier in order to protect us from that particular situation, lest it become more that we can bear! Think of Job...how that, though God allowed satan to afflict him, satan was restricted in that affliction.
In our infirmities, discomforts and pressures of life, God gives sufficient grace for as long as the trial endures...for as long as it takes, God raises a barrier between us and the enemy. I am reminded of how God placed the Cloud of His Presence between Israel and Pharaoh's army; how it became light to Israel and darkness to the Egyptians. How we see God in our circumstances depends on which side of God we are on!
God did not remove Paul's thorn because in it was a lesson for the learning. God's strength is displayed through our weaknesses...it's for our good and His glory!
We may rely on God's Sufficient Grace in our times of need.
A favorite hymn of mine is "Day By Day", written by Carolina Sandell Bell. Sing it throughout this day as a precious reminded of God's Sufficient Grace!