"Let not your heart be troubled: ye believe in God, believe also in me." John 14:1
The answer: "Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee." (Isaiah 26:3) When we keep our mind (also means heart) stayed on Him/the Lord, which doesn't mean that we are just to constantly think about the Lord and nothing else, but the word means to have our mind firmly leaning up Him. When our hearts/minds are firmly leaning/trusting upon Him, He will keep us in not just peace but perfect peace. Perfect peace means to have completeness, soundness, quietness and contentment. When our hearts/minds are at rest, content, whole and well, they are not troubled.
So how do we keep our minds on the Lord? "And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God." (Romans 12:1) When you are tempted to be troubled in your heart, renew your mind. Renew it in God's Word. Combat those thoughts that would make you anxious with His Word. Capture those thoughts that make you afraid with His Word. Counteract them with His Word. His Word brings life, health, peace, joy, contentment, etc. As you are transforming your mind with God's Word, you are at the same time keeping it stayed on Him because He is the Word. (John 1:1)
Have a great day. Don't let your heart be troubled. Keep it on God.