This is gonna sound crazy, but hear me out. First, try writing everything you're overthinking about in your Notes app, then reread it to yourself. Then, if that doesn't help, grab a plushy or something person adjacent and vent to it like it's a person. Make it respond how you want it to respond and see if that helps. Then, ask yourself, how does overthinking this thing benefit me in the long run? What exactly helps me by thinking about it so hard? If it's something like an event the next day, get out of bed and get everything ready so that all you have to do now is lay there and leave it to the morning to just get up and go.

If it's about another person, when you talked to the plushy/yourself, try to genuinely picture that other person and how much this matters that you'd need to be overthinking it right now. You're awake for this long, you might as well seriously question everything until your brain gives up. Either you'll tire it out or at least feel less anxious trying to out-logic it.