We've all heard that phrase a time or two; in fact, someone even wrote a song by the same title!
Don't worry; be happy. There's a lot of wisdom in that little phrase. I Peter 5:7 & 8 says, "Cast all your anxiety on Him because He cares for you. Be self-controlled and alert."
Why do you think that verse ends with be self-controlled and alert? I think I have an idea. Worry can be toxic not only to ourselves but to others, especially if in our worry we grumble, complain, and fret aloud. Our attitude of worry can release itself to others and give off an almost poisonous stench, pushing people away from us. Instead, we need to project our worry toward God, the One true source of comfort, encouragement, and peace. Today has enough struggles of its own without trying to figure out tomorrow! Check out Matthew 6:34: "Therefore, do not worry about tomorrow, for tomorrow will worry about itself. Each day has enough trouble of its own."
If worry snags hold of you, gripping you from the inside out, cast it away in Jesus' Name. It is not of God! Worry is a stronghold that can drag you down a path of doom and darkness. On the other hand, don't beat yourself up if it sneaks up on you. Simply cast it aside again--and again if needs be. God understands your human weakness, and He wants to help you with your tendency to worry. He awaits your attention, so turn your thoughts toward Him whenever worry begins to take hold. He INVITES and DELIGHTS in your dependence on Him, especially in those times of struggle.
Don't worry; be happy. Perhaps easier said than done, but with prayer and practice, you can find the kind of joy you desire to make it through the rough times. GOD IS FAITHFUL, AND HIS WORD NEVER FAILS.