100% I'm an overthinker by nature too, from the minute I wake up until I eventually pass out but it's either part of your nature or I don't want to assume anything but you may have had past trauma that led you to feel this way and then the state of the world just brought it all down at once but keep in mind, people have lived through wars and watched their entire family and friends die, so try your best to put things in perspective and appreciate what you can, don't take it for granted and try not to torture yourself about things that have not happened (yet or hopefully ever) A lot of it is retraining your brain but it takes consistency of "catching yourself" overthinking and redirect those thoughts to the current moment. If you can literally shut off those thoughts and stay in the moment, you will see that those feelings will subside.
For me now it's like a light switch.. as soon as I catch myself dwelling or overthinking, I stop and think about what I have to do next in the current moment but the key is to catch yourself early because once you fall down that rabbit hole, it's hard to get out. I'm sorry you're going through this.. I really do understand