Overthinking occurs when one has been upset or is not living in the present moment and can amount to unhealthy rumination involving self-doubt, regret, guilt, anxiety and all the usual suspects of negative emotionality and mental states.
Overthinking arguably stems from a place of inner insecurity and fear rather than a place of strength, self-belief and self-trust.
Overthinking is different from mindfulness which is in tune with Being as overthinking usually involves data that relates to the ego rather than the true self.
People who are unsure about themselves over-think and get worse as a result when the goal is to be mindful of the whole through getting to grips with truth, that which is.
Present moment awareness - being engaged in the now whilst being aware of the whole - is arguably the greatest antidote to the vicious demon that is overthinking.
At least for the philosophers among us.