To think is a natural ability of the human brain; the greatest gift to humanity from nature. This ability is meant to analyze the situations and respond appropriately to move forward in life. It is your thought that creates your reality, so you have to use it wisely. However, if the ability to think gets misused by overthinking, it becomes detrimental to your professional performance as well as personal well-being.
Overthinking basically means to think too much and it applies either when you think more and act less or exaggerate the problems by thinking worst all the time. Overthinking is a common problem and almost everyone at some point of time felt doing it more or less. Although it is not wise to act without thinking, but overthinking interfere with a needed action by making you unable to take decisions.
You cannot stop overthinking altogether; after all you have to think to respond to the situations with a best possible alternative. Overthinking in itself is not a problem; in fact, it is a good exercise for brain, but when overthinking halts your progress in life and start ruining your relationship, you need to address it.