Recognize the negativity
You may just be having a sense that something is wrong, but not actually know what it is. Go find out. Do some investigation; whether it's searching inside yourself or asking trusted people you know. Become aware of just what the negativity is, what it looks like and especially what it's doing to you.

See if you can stop it at its source
Once you can recognize the negativity, see if you can get rid of the cause. It could be a person, or an activity, or an old habit. Whatever it is, see if you can make it go away.

Fight fire with fire
In lieu of removing the source, you can always control its effect on you. Literally fight negativity with negativity… Say "NO" ... if not to the source, at least to yourself. You may not want to get too loud if you're in the library or on a crowded street, since people might look at you funny, but whenever you feel the negativity creeping in, say it. "NO!!!" can have a powerful way of stopping it from sinking in further.

Fill 'er up with the good stuff
If you're filled up with positivity… good thoughts, well wishes… it will be tougher for negativity to make room. After you say "NO"… immediately picture what you DO want (whatever that is, even if it's seemingly unrelated to the situation) Say "YES." For example, you might have an argument with a coworker or spouse and you picture a vacation home in southern France. Seems off-the-wall, but it shifts you towards a better state of being.

Stay true
All too often, we get into a revenge mode, or a "well, everyone else is doing it" mentality… and we let that justify our slip into a darker place. Don't do it. Just don't. If you truly want the world to be a better place, then you have to be the better person. Don't make "Everybody does it" a true statement… as long as you don't do it, the world is automatically a better place.

See how important you are?