You need to get rid of those negative thoughts, that's whats causing your negativity. They can be a result of something someone has said to you, a negative idea that you found and you can't stop thinking about it, or maybe its frustration for whatever reason and you are manifesting it.
You need to clear your mind.
Sit down in a relatively comfortable and distraction-free place. Listen to your thoughts, pay attention to how you feel. How bad is it?
Now you need to turn it off. Meditation works.
Close your eyes, and focus on the sound of your heartbeat, breathing... anything. Your mind usually needs something to do, that's why you focus on something external. By doing this you are calming yourself, and training yourself to control your thoughts.
This is something you do regularly. 10-15min a day is good. If its really bad, do it more often.
Now you need to think positive. Look at the bright side of the situation whenever you can, and when its appropriate.