Pain is the Platform: For Radical Redemption and Purest Praise

2 Corinthians 12:1-10 (ESV)
1 I must go on boasting. Though there is nothing to be gained by it, I will go on to visions and revelations of the Lord.
2 I know a man in Christ who fourteen years ago was caught up to the third heaven—whether in the body or out of the body I do not know, God knows.
3 And I know that this man was caught up into paradise—whether in the body or out of the body I do not know, God knows—
4 and he heard things that cannot be told, which man may not utter.
5 On behalf of this man I will boast, but on my own behalf I will not boast, except of my weaknesses—
6 though if I should wish to boast, I would not be a fool, for I would be speaking the truth; but I refrain from it, so that no one may think more of me than he sees in me or hears from me.
7 So to keep me from becoming conceited because of the surpassing greatness of the revelations, a thorn was given me in the flesh, a messenger of Satan to harass me, to keep me from becoming conceited.
8 Three times I pleaded with the Lord about this, that it should leave me.
9 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.
10 For the sake of Christ, then, I am content with weaknesses, insults, hardships, persecutions, and calamities. For when I am weak, then I am strong.

“Harass” here is: kolaphizo
1. to strike with the fist, give one a blow with the fist
2. to maltreat, treat with violence and contumely

It is an experience of pain.

What does "perfect my power mean"

The word “teleo” means completed, fulfilled, accomplished, finished

Weakness, “astheneia”, which is usually:
* want of strength, weakness, infirmity
   a. of the body
       i) its native weakness and frailty
       ii) feebleness of health or sickness
   b. of the soul
        i) want of strength and capacity requisite
            a) to understand a thing
            b) to do things great and glorious
            c) to restrain corrupt desires
            d) to bear trials and troubles

Pain creates a context and culture where the Lord's power accomplishes or completes the will of God.

That power completed in the context of weakness is shown in vs 9 & 10.

2 Corinthians 12:9-10 (ESV)
9 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.
10 For the sake of Christ, then, I am content with weaknesses, insults, hardships, persecutions, and calamities. For when I am weak, then I am strong.

Strong here means: strength to magnify the Lord and his kingdom.