What can I do to stop overthinking everything?

Thoughts are like Insulin

The pancreas secretes insulin to pull sugars out of the bloodstream before they become too concentrated and toxic. Your pancreas does not think about releasing too much or too little it releases based on the need as indicated by another biological response too complicated for this explanation. Your brain is an organ, just like the pancreas. You are not your brain, you have one, and it works by secreting thoughts. Yes, thoughts come from a life lived and experiences to draw from. Your brain is doing exactly what your genetics and environment taught you to do for your survival. This was necessary for your previous relationship. 

Sometimes we say things that indicate we believe something is right or not. Often these things are based on a preference that is from our mind as well, that says this or that would be better. Your mind is the problem in the oversecretion of thoughts and is also the same mind telling you to ask this question. The funny thing is, when you say what you did here in the question, you trusted the mind to give you the proper information to properly combat the way it is. You can see how problems continue to cycle over and over again. We believe our mind, even though we say it is the problem. 

What you can start to do with beliefs (such as overthinking) is to identify that thought. You can separate from what your mind tells you by acknowledging thoughts and that these thoughts get your attention. It isn't that your mind is against you, but it wants to keep things the same way because our body likes homogony. Your thoughts then are familiar to you, and even though you deem them as disruptive now, at some point, you learned to overthink, which kept you safe and aware. 

What to Do

You can check the belief that you would be better if you thought less. As though your mind isn't your mind but someone else's mind. We perceive other people to be doing life right and us wrong. It isn't your overthinking; it is that you believe by stopping thinking that you would be better. Maybe, the way you think is a gift, but you perceive it as a detriment. Why is it a detriment? How is it interfering with life? These are the questions you have to ask yourself to understand why you want this way about you to go away or be "fixed." 

Mindfulness is the key to all brain-related questions. You will not have a different mind, no matter how hard you try. Your mind will do what it does. You can practice noticing your experiences and thoughts and sitting with them without reacting or judging them. You can start to separate from the thoughts that captivate you and continue to do what you want to do in the process. 

Do not do what many people do in therapy; get lost in their thoughts and indulge the belief that they won't be better until they remedy whatever their thoughts tell them are the problem. You had to overthink to ensure safety; it is a "good" thing. You now need to know how to manage this "good" thing and allow yourself to let some thoughts go without thinking you need to acknowledge and remedy all of them. Therapy for someone like you can help get over any internal struggles of self-perception from life, but acknowledging you are an overthinker, know that you will probably be fed thoughts constantly. 

Do not remedy these thoughts, but let them pass by. You are OK if you let thoughts pass. Have faith in doing what matters to you, not allowing these thoughts to interrupt what matters. Also, do not define your peace by your number of thoughts. You can have peace and a lot of thoughts. You can live in this dichotomy.