Think of something bringing you down today. Just pick one thing interfering with you being in a great mood right now. If you're feeling great today, think of something that put you in a bad mood several times in the past.
Just pick one thing. Do this now... You have time (this is a short article).
Okay, now see if you can think of one thing you believe about it. What do you think is true about it?
Now ask yourself throughout the day today, "Am I SURE that's true?"
For example, John has a pain in his knee and it is bringing him down today. He believes it's a sign he's getting old. So he spends the day asking himself, "Am I SURE it is a sign I'm getting old?"
Try this today. It's a simple technique. You can do it while doing other things, so it doesn't need to take much of your time.
It is possible asking yourself if you're SURE it's true will make you feel uncertain about your belief. This is a good thing. The uncertainty will make you feel better. You might be surprised by that, but part of what's bringing you down is a negative assumption you're making, and when you put that assumption into question, the depressing effect of the assumption is eased and lightened, so you feel better.
Even if you think you're sure about your assumption to begin with, keep wondering, "Am I really sure that's true?"
Just wonder. Don't try to think positive. Don't try to make yourself believe something. Just wonder about it in a curious, interested way. I think you'll be surprised at how effective this very gentle technique is at improving your mood.
Learn more about this technique: Antivirus For Your Mind