Christians have joy because - The joy of the Lord brings mental and emotional healing:
Psalm 94:19 - When anxiety was great within me, your consolation brought joy to my soul.
Two opposing thoughts cannot occupy the mind at the same time. Perhaps the greatest antidote to anxiety is a trusting relationship with Jesus our Lord. My prayer every night is that God would grant my children the gift of faith. It is the greatest gift in the world. Trust of course is built strong over time. I can tell you however that Jesus is more than able to prove Himself to you if you will only let Him. Do that and you are in for the most exciting ride of your life, for the rest of your life.