[Isaiah, speaking to God in praise]
"You will keep in perfect peace
him whose mind is steadfast,
because he trusts in You."
Isaiah 26:3 (NIV)
You may find that those times in your life that were without peace, without rest, were not because of a lacking on God's part to grant peace. Instead these times may be linked to your own lack of steadfastness of mind.
Trusting in God is not a one-time thing. You don't choose to trust God at salvation, then never have to make that choice again. Rather, Salvation marks the beginning of a life that is devoted to trusting God. And this choice to continue to trust God is a moment by moment choice. It needs to be made again and again.
It is in our darkest hours that we are the most in need of peace that we need to keep our minds steadfast and trusting in God.