Go to whichever mirror in your house that has the best lighting. Most likely gonna be the bathroom mirror, but if you have a decently sized and well-lit hallway mirror then that's even better.
Once you're there, look at yourself. Smile, and try to give off a positive expression even though you're feeling down. Now say to your mirror self, with energy and confidence; "you are a great person."
Now say it again, preferably with some variation between each time ("you are a great person," "you are a great person," "you really are a great person," etc.). Chances are the variations will come naturally. The only real trick is to make it sound like you really mean it.
Feel any better?