I know it seems ridiculous, but it really does work! That is kind of what cognitive-behavior therapy is all about.
It takes effort, but it is well worth it. Not that you should be oblivious to the realities of the world, but why be depressed/negative when it doesn't help?
This is my best understanding of how it works: basically when you counter a negative thought by ignoring it or trying to think of something positive instead, it actually creates changes in your brain! Think of it like every time you reinforce a thought or behavior you are carving out a pathway in your brain to make that thing easier and easier the next time and the time after that.
It really is all about the connections you are forming in your brain and how different neurons are firing.
I think the key to success with changing your thoughts is repetition.
That is why people with depression have the best outcome if they COMBINE meds with therapy. The therapy actually does stuff to your brain!
In fact, pretty much everything we do changes our brains.
Anyway, if you are constantly repeating negative thoughts to yourself it will just reinforce them and they will be easier and easier to have. If you repeat positive thoughts, same thing.