There are quite a few things you could do to help this. A few small things to try and keep in mind are perspective (e.g. is this actually a big deal? Will I forget about it next week?, etc.) and if there are solutions to troubles try and carry them out. Also, it's important to understand that a lot of the time, overthinking focuses on catastrophic outcomes, which usually don't actually happen. Try and be mindful of where you are currently and what you can do right now.
Finally, be kind to yourself. Most things aren't worth making yourself ill over with stress.