Sometimes I just hold my breath and try to kind of 'pause' for about 15 seconds or so, just allowing my mind to basically empty. It sounds cliché, but it really works for me, helps me wind down and then everything is a lot easier to handle. By the time I breathe again, I end up with the mindset that very little in life is worth being upset over.
It helps to do something afterwards, too. Partake in what is considered a mindful activity, like reading or playing a video game, whatever your thing is. Just remember that nobody has ever actually benefitted from any sort of worrying, it is a complete waste of time. You will feel so much more powerful in life if you can learn to stay on top of your own problems and negative feelings. You can't enjoy life if you let it control you, because life is a bitch and will fuck you blind if you give her the chance.