The verbs in the first few verses of Psalm 37 (one of my favorites) stand out to me:
Trust in the Lord
Delight yourself in the Lord
Commit your way to the Lord; trust in him
Be still before the Lord
Wait patiently for him
Fret not yourself over the one who prospers in his way, over the man who carries out evil devices
Refrain from anger, and forsake wrath
Fret not yourself; it tends only to evil.
The repetition of “fret not” indicates the Israelites were in a situation that could cause them to fret, namely, the encroachments and threats of the wicked. Later in the chapter God assures them that He will take care of them, provide for them, protect them. Their faith was not passivity nor naiveté, not sticking their heads in the sand: rather, it was characterized by active trust, patient waiting (v. 7), and focusing on doing good to others (v. 3).
Peace is a part of the fruit of the Spirit (Galatians 5:22-23), but we’re also to “keep in step with the Spirit” (v. 25). We can work against peace of heart by fretting, magnifying the problems, spending too much time with swirling, fearful thoughts. Or we can work with God to promote peace of heart by focusing on Him, committing our way to Him, delighting in Him, trusting Him to take care of the issues, and getting out of our own heads to see what we can do for others.
It’s counterintuitive to pray for or expect peace of heart without taking the means God provided to take our thoughts captive. When we find ourselves fretting, fearful, downcast, we seek God and remind ourselves of His truth in His Word.