How can I stop overthinking everything and be satisfied?.
This happens with everyone. This is a good sign if you have enough time to spend on thinking. This means you are open to learning new things which is a good quality. However, you should ensure such a habit does not cause harm to your professional life. Creative people suffer due to this habit. If suppose, a creative person is asked to create a powerpoint presentation of 10 slides which should be ready within 2 hours, he may take 2 days. Creative people are not easily satisfied. They keep on correcting themselves. It is good in the long run because they will become the expert on this topic but the employer cannot afford to give excess time to their staff for one single task.
Overthinking everything on a subject in which you are pursuing Ph.d is okay but otherwise this habit will ruin you.
Judge yourself and draw a line. Tell yourself that its over now and let me concentrate on something different. Variety is essential in very sphere of life.