I know this is not the usual kind of question for this forum but I did not know where else to post. Found out 3 weeks ago that my partner of 4 years had been cheating. I am not even angry anymore, just stuck in my own head 24/7. Replaying conversations, looking for signs I missed, wondering what was wrong with me. It is exhausting. How do people actually get through this? Does the overthinking ever stop or do you just learn to live with it? Genuinely asking.
It stops. Not overnight, not in a week, but it does stop. I went through something similar about 2 years ago and I remember lying awake at 2am running through every single conversation from the last 6 months trying to find the exact moment I should have noticed something. You know what that gets you? Nothing. Just tired. The brain does this because it is trying to make sense of something that does not make sense. Betrayal breaks your model of reality a little bit and your mind keeps trying to fix it by finding the missing piece. There is no missing piece. The problem was never you. Give yourself time and please stop looking for answers in the past because they are not there.
Three weeks in is still very raw. Be easy on yourself right now. The overthinking at this stage is almost a physical response, not just emotional. Your nervous system is in alert mode because something that felt safe turned out not to be. So your brain keeps scanning for threats. It is basically your mind trying to protect you from being blindsided again. The way through it is not to stop the thoughts by force but to slowly rebuild a sense of safety. That comes from routine, from people who show up for you consistently, and from small wins that remind you that you are okay. It takes time but the frequency of those intrusive thoughts does reduce. Promise.
okay real talk 
I handled this the absolute wrong way when it happened to me. I went full detective mode. Went through everything, asked every question, demanded every detail. You know what all that extra information did? Made it worse. Way worse. Every new detail just gave my brain more material to replay at 3 am. I am not saying do not process it. Process it. But there is a version of seeking answers that turns into self torture and you have to recognize when you have crossed into that zone. At some point, you have to make a conscious decision to stop feeding the spiral. Easier said than done but that decision is actually the turning point.
What helped me personally was writing. Not journaling in the cute notebook kind of way, just opening a notes app and dumping everything out of my head. Every thought, every question, every angry thing I wanted to say. Get it out of your head and onto a screen. Your brain keeps looping on things partly because it is afraid of forgetting them or not processing them properly. When you write it down, you are essentially telling your brain: okay I have logged this, you can let it go now. It sounds small but it genuinely reduced the mental noise for me after a few days of doing it consistently.
DataBeacon, something nobody told me when I went through this is that the overthinking is not really about the cheating itself after a while. It shifts. It becomes about your identity. You start asking things like who am I now, was any of it real, do I even trust my own judgment. That is a different problem and it needs a different kind of work. Therapy helped me a lot with this part specifically, not because the therapist had magic answers but because having a weekly session gave me a structured place to put all the heavy stuff instead of carrying it around all day. Even if it is just a few sessions, it is worth trying.
can I be the slightly sarcastic one here 
the fact that you are asking how to stop overthinking means you have already hit the wall where you realize the overthinking is not helping. that is actually progress. most people spend months in the spiral before they get to that point. so weirdly, you being here asking this question is a good sign. the spiral is not going to last forever. you are already looking for the exit which means you are going to find it.
Adding to what Fluxorix said about identity, I think this is the part that people underestimate the most. When you are in a long relationship you build your sense of self around that bond. Your plans, your routines, your idea of the future. When betrayal happens it does not just end the relationship, it destabilizes that whole structure. So the overthinking is partly grief. You are grieving a version of your life that you thought was real. That grief is valid. You are allowed to sit in it for a bit. The goal is not to rush past it but to move through it without getting stuck permanently.
I read something once that stuck with me. It said something like: you cannot think your way out of something you felt your way into. The betrayal hit you on an emotional level and no amount of mental analysis is going to resolve it. You have to feel it, let it move through you. That does not mean wallowing. It means giving yourself permission to be sad, angry, confused, whatever comes up, without immediately trying to figure out why or fix it. The thinking slows down when you stop using thought as a defense against feeling.
Practical stuff that actually worked for me during that phase: I set a literal timer. 20 minutes a day where I was allowed to think about it as much as I wanted, go over everything, feel all of it. Outside of those 20 minutes, every time the thoughts came up I would acknowledge them and redirect. Something physical helped a lot too, running, gym, even just a long walk. Your body is holding stress and it needs somewhere to put it. Also deleting or muting anything on social media that was connected to that person. Not dramatic, just practical. The less ambient exposure you have to reminders the faster the brain starts to settle.
This might sound sideways but it helped me to focus on what I actually knew for certain versus what I was imagining. Because a huge part of overthinking is filling in gaps with worst-case stories. You find yourself building entire narratives out of half a conversation or a vague memory. Try separating facts from stories. Write two columns if you want. What I actually know. What I am assuming. You will probably find the assumption column is a lot longer. That does not make your feelings less valid but it stops the spiral from going places your mind invented rather than places that actually happened. Also if it helps at all, a lot of people use apps like Xnspy in the early stages just to settle specific factual questions they are stuck on, and then close that chapter and move forward. Sometimes certainty, even painful certainty, is less exhausting than endless wondering.
Something that helped me that nobody really talks about: I stopped trying to understand why they did it. Because there is no answer to that question that will make you feel better. None. You can get the full explanation, the whole breakdown of how and when and why, and you will still feel exactly the same. The reason is that the explanation does not fix what broke inside you. What fixes it is time, self-care, and slowly building back trust in yourself and in your ability to be okay. The why is a trap. Do not go down that road.
Coming in late but wanted to add one thing. Everyone in this thread has given solid advice. What I want to say is: be patient with yourself in a real way, not just as a phrase. That means on the days you feel fine, great. On the days you wake up and it hits you all over again, that is also okay. Healing is not a straight line. You will have good weeks followed by a random Tuesday that knocks you sideways for no obvious reason. That is not regression. That is just how emotional processing works. The overall direction is still forward even when individual days feel like going backward. You are going to be genuinely okay.
DataBeacon, I just want to say: the fact that you are not angry anymore and mostly just stuck in your head is honestly a sign that you are already starting to shift from the acute pain phase. Anger is the first wave and it is loud. The quiet overthinking stage is uncomfortable but it is also closer to the processing stage. You are doing the work even when it does not feel like it. The thoughts will space out over time. One day you will realize you went a whole morning without thinking about it. Then a whole day. Then it becomes something that happened to you rather than something happening to you. That shift comes. Hang in there 