anything - try and think about whether there is anything you can do about the situation in a given moment. is there anything you can do right now that will make you feel better about it? if not, then what's the point in worrying? leave all your troubles where they belong, and don't let a bad yesterday (or a potentially bad tomorrow!) ruin a good today.
i find that whenever i start feeling a little stressed out, it really helps to just write down all my thoughts and emotions in a journal. i often start worrying about something, then end up imagining all the worst scenarios and get really caught up 