Isaiah 26: 3, said, “You will keep him in perfect peace, whose mind is stayed on You, because he trusts in You.” Focus on God, and He will keep you in perfect peace, because if you stay focused on God, you will trust Him; which is the main thing.
I guess the simple interpretation of this verse is, ‘All you have to do to get perfect peace (not fear, or anxiety, or worry, or depression, or anger, or frustration, or guilt, or hatred, or any of those other negative emotions that keep crowding in on us, every day and night) is to keep your mind stayed (focused) on God. That means every time the devil attacks, you say, “Get away from me in Jesus’ Name!”; and you turn your focus back to God.
But it doesn’t sound like focus is the only ingredient. The real primary ingredient is trust. Focus is something you do with your head (your mind, your brain); trust is something you do with your heart, and God looks at your heart. You can focus all you want, but because it is something you do, it’ll never give you perfect peace. But if you trust (with your heart), God sees that trust in your heart, and He keeps you; He keeps you in perfect peace, because He is responding to the trust He sees in your heart. So trust is the miraculous, supernatural key ingredient that draws man to God, and God to man. And what are you really trusting in? That “God so loved the world, He gave His only begotten Son, that whoever believes (trusts) in Him, will not perish but have everlasting life.”