Honestly, I am the queen of overthinking. You could tell me something and over the course of 5-10 minutes I could go from believing you to fabricating some sort of alternate statement that means completely the opposite of what you had intended to say at all. My personal favorite is when I overthink the words between the lines because not only did I infer what you were thinking while you were talking, but I also sat there and twisted around both my inference and what you actually said out loud.
I won't lie. Working on this bad habit has been a personal struggle of mine. It's taken me years to realize that if you really trust someone then you should believe them when they tell you something. You shouldn't sit there and come up with reasons why what they're saying can't possibly be true. And if you feel like there is something you need or want to know that isn't being said, rather than assuming what they are thinking, ASK. If they seriously get mad about that then you deserve better. If they would really rather have you sit there in your own assumption-induced agony than tell you the truth, then they obviously don't care enough about you.
The reality is, overthinking pisses people off. And for good reason. That same crappy feeling you get while you're sitting there second-guessing is the same thing they feel when they know they've been second-guessed. No one wants to feel like everything they say is being over-analyzed, and the more you do it the less they are going to want to talk to you about anything. It isn't fair to either of you, and it sure as hell isn't going to make the situation any better.
Sometimes you have to take a step back and as yourself if the thoughts you are having are justified. Have they actually said or done something that you honestly think is evidence to support what you're thinking? And if so, approach them about it. Odds are it could all just be a misunderstanding. And if you are honest with the person about your tendency to overthink they will probably understand, so long as you don't do it so frequently it drives them insane. So give your brain a rest and stop overthinking. And if you're currently overthinking about overthinking as a result of reading this post - contact a medical professional immediately because you've got bigger problems honey.