We do not know what Paul's thorn in the flesh was (see Numbers 33:55; Ezekiel 28:24; Hosea 2:6). We only know it was meant to keep him humble, to burst his pride.
It was a messenger from Satan, meaning something or someone evil, so Paul asked the Lord to remove it. The Lord replied each time, "My grace is all you need. My power works best in weakness." Paul's weakness allowed Christ's power to work through him by God's grace. Paul accepted his suffering because Christ's strength showed through his weakness.