How many thoughts do you think in a day?
The answer is a lot. We are addicted to thinking and re-thinking and re-re-thinking until we tire ourselves out. At that point we go to sleep, wake up, and restart the process again.
I'm not zen enough to give you an antidote to re-thinking. I struggle with overthinking just like everybody else. Logically, I understand you can't out-think some problems, yet the very nature of this truth prevents me from turning thinking 'off'. I could tell you something like, "thinking robs us from the present moment", but that's no more actionable than a fortune cookie.
A tactic I've found to be effective in reducing re-re-thinking is closing open loops.
Open loops are thoughts you know to require eventual action but haven't yet committed action to. Calm these thoughts by giving them direction: they need a job to look forward to, else they misbehave and demand your precious attention.
Open loops can be found hiding within general anxiety because anxiety is often a side effect of uncertainty. "What am I going to do?!" is often the open loop found at the root of anxiety. Close the loop by answering the question. Calmly explain to the loop how you intend to handle the situation, whether it be nerves before a date or a big meeting with your boss, or when you promise to give it the attention it deserves.
I sleep best when I close my open loops before bed. I scribble down the thoughts asking for my attention, moving them from mental RAM to removable storage in the form of a sticky note. This transfer of thoughts from my head to another medium is usually enough on its own. The open loops in my mind just want to be noticed. They take comfort in knowing the issue they represent hasn't been forgotten.
Answer your anxiety by closing open loops.