My life slogan is "It happens". When I get a crappy grade on an exam, I don't linger on it and let it get me upset because "It happens". You just have to learn to move on. In the grand scheme of things, 99% of things that are upsetting don't really matter in the long run. There is too much to be happy about and to laugh about.
We live from challenge to challenge, day in and day out, and things are hard. But just blast your favorite song and surround yourself with people and things that you love, and those challenges won't seem all that bad. Bobby McFerrin said it best, really..."Don't worry, be happy".