A common cause of overthinking is that you’re confused, so you keep trying to do better, but you’re not really sure what’s better, so you stay confused.
If you’re doing something that you don’t know how to do, you may keep retrying unsuccessfully. Then you may do a bunch of thinking to try to fix it, which often doesn’t work because you simply don’t know how and thinking about it more won’t fix that.
You may spend a lot of time trying to make improvements to your writing, or to how you do a skill, but you don’t actually know what is good or bad, so your changes are aimless. Each time you make a change, you still aren’t sure if the result is good (or even if it’s better than before the change), so you keep making more changes hoping something will stand out as good to you.
If you can’t evaluate success and failure well, then you can work on something endlessly but never know when it’s done. In that case, roughly half of your changes will make it worse instead of better, so you’re not making progress.
If you aren’t sure about which changes are improvements, you need to aim for some other (simpler) goals that you understand better.
If you can’t judge success and failure for what you’re doing, don’t do it. You don’t know if you’re wasting your time, and it’s easy to get stuck, overthink things, analyze at length without reaching clear conclusions, etc.
If you want to work on a topic that you don’t know much about, you should set goals that you understand. Instead of succeeding at or within the topic, you can have a goal like exploring the topic. Then you can do some activities (like reading and watching things) and confidently evaluate whether you succeeded at your goal. If you read a few things and know more than before, then you succeeded at a generic exploration goal. You can also set more specific goals like to read a book (or better, to try reading it and finish it only if you like it) or to find out what a term means.
You don’t have to evaluate ideas within or about a topic in order to do an exploration-based goal. Adults and teens can always make progress using only goals which they can competently evaluate success or failure for (we don’t know in detail what infants do). If you want to reach a conclusion about a topic, or write a good article about a topic, then you actually need to understand the topic and be able to evaluate what’s correct. But you can begin working on a topic in ways that you already understand.
Choose (immediate) goals that fit what you know how to do now, which you can evaluate success or failure for.
You can also have longer term, overall or big picture goals goals, like to become good at a skill or topic, even though you don’t know how to do them now. However, at all times, you should have an immediate goal that you’re working on, which you can evaluate success or failure at. A big goal can be divided into sub-goals, which can be divided into sub-sub-goals, until you get to small parts that you understand how to work on and evaluate whether your work was successful. It’s never efficient learning to be working on something (as your immediate goal right now) without being good at judging success and failure.
Evaluations should involve rational confidence, not arrogance nor feeling unsure. It’s important to have good judgment about when you are and should be confident based on your knowledge, and when you don’t really know something.
Overthinking often comes from working on goals where you’re not really sure what success is. A common pattern is to try harder, but that doesn’t solve the problem, so you try harder again, which still doesn’t work, and so on.