What does it mean to be "hemmed in" by God? A hem is sewn into fabric to keep it from unraveling, fraying, losing shape and falling apart. A hem makes a garment exactly the right length.
As I reflected on this verse a few weeks ago, my mind went to "In your book were written, every one of them, the days that were formed for me, when as yet there were none of them" (Psalm 139:16), and then to Psalm 16:5-6 - "The Lord is my chosen portion and my cup; you hold my lot. The lines have fallen for me in pleasant places; indeed, I have a beautiful inheritance."
God has hemmed me into this day. The mistakes of the past are secured behind me, covered by Christ's blood. The unknowns of the future are secured before me, resting in His trustworthy, sovereign hands. I have only to live today, here. Today is a day He formed for me before I was even born--and even better, His hand is on me today!
He has sewn up the boundary lines so that this day doesn't have to unravel, so that I need not have frayed nerves, so that I don't fall apart. He has expertly tailored this day--the right length, the right shape. I have only to trust Him in this; to cling to the hand that is upon me, that upholds me; to praise Him for His sovereign, faithful care; to thank Him for the present and realize that He is Present.
[see also: hemmed = besieged]