If you catch yourself forlorn and worried (yes, you must be aware of your present mood), ask yourself just this one question: "Am I thing about the outcome/result of something"?
Invariably, man's prime of cause of worry is what is going to happen-right? If your answer is 'yes', then, you will notice that you will be contemplating all sorts of negative results of your recent action which may be your conversation with your immediate family or co-workers in your office, your recent purchase of a gadget or a book etc.
Once you notice the nature of your contemplation, stop and ask yourself why you are thinking on the negative directions. This will reveal to you many things. Mostly, you will find yourself holding on to something that had happened in your past that ended up negatively and you firmly believe that the same thing will happen again.
Why? Why it should repeat? There is no such law of universe that things will happen the same way for any given issue!
Realize and accept this fact and you will catch yourself thinking on positive results. Immediately, your mood will perk up and cheer will return to you.
See, even before something good happened, you are happy! So, it is all in your way of thinking. Thinking positive itself will make you happy. Why not think positive!