My former MD always use to espouse the view that 'the mind completes the picture'. I stand by this and believe it has served me well both in life and at work.
We talk to ourselves more than anyone else, usually (though not always) as quiet internal dialogue. Try to take a couple of minutes throughout the day to stop and check what these messages are and identify what it is you are telling yourself, and why?
Ruminating on the problems and negatives, perceived or otherwise keeps you firmly rooted in that place rather than focusing on the present or planning for the future.
Mindset shift is required to change your inner dialogue, and it takes work, but the rewards are worthwhile and will impact positively on your outlook, and reflect how you feel about yourself and those around you.
Overthinking in this way is called rumination. While we worry about what might occur in the future, we ruminate about events that have already happened. A ruminative reaction to an event often triggers memories of similar situations from the past and an unproductive focus on the gap between the real and ideal self.