I'm no psychologist, but a good way to stop anxiety brain and ruminating is to interrupt the thought cycle. Meditation and mindfulness are good ways to do this by turning off the thoughts and resetting.
There are plenty of apps out there for this but a lot of them are paid. If you're just starting out or don't have financial resources, I highly recommend the 'first this' podcast by Kathryn Nicolai. At least in my podcast feed it's free and no ads, each ep is 10 mins of guided meditation. It helps my brain calm down and shut up.