"And which of you by being anxious can add a cubit unto the measure of his life? If then ye are not able to do even that which is least, why are ye anxious concerning the rest?" (ASV)
Don't worry. Such a simple thing to say, but not as easy for many people to do. It's far easier to get caught up in the stress of daily living and fret over things as if we have some kind of control over them. Christ told us that we can't add even a day to our lives by worrying, but now we know that not only can't we add, but we may in fact shorten our lives through it.
The root of worry is a desire for control. We want to control the outcome of a situation. We want to control what people will think of us. We want to control our lives. This desire is based in sin. A righteous man doesn't desire control, rather he relinquishes it. When we allow God lordship over us, we give up all control and power in our lives to Him. We become followers rather than leaders. If we are still struggling for control, we are working against Him.
The reason many people have trouble giving God complete control over their lives is they don't trust Him to do what they want. We look at immediate situations and we know how we want the outcome to go, but we don't know what God's plan for us is. We forget that while He may not make everything work the way we want it to, He WILL make everything work for our betterment. We allow our faith in His goodness and His love for us to be shaken.
How then can we defeat the worry that plagues so many of us today? We need to remind ourselves of three things. The first is that God alone is sovereign. He is in control, and that is His right. The second is that God loves us. He wants the best for us in all situations. The third is that He has never let us down before. While He may not have done things the way we wanted them done, if we objectively look at all the past situations that we've given over to Him, we'll see that He has always been faithful. Armed with this knowledge, we can finally give our worries over to God and let Him reign in our lives.