PESSIMISM IS A WAY OF THINKING. It is a way of thinking that is unnecessarily self-defeating and counterproductive. And, most importantly, pessimism is an inaccurate way of thinking. If a thought in your head is accurate and not self-defeating or counterproductive, then it is not pessimism, even if it might be considered a "negative thought."
What kinds of inaccuracy am I talking about? Thought-mistakes, technically known as "cognitive distortions" like overgeneralization and black-or-white thinking are inaccuracies. The thoughts and beliefs are objectively untrue.
Pessimism is a way of thinking about an event that mistakenly makes the circumstances out to be more depressing or disheartening than they really are, sapping your motivation and taking away your will to try.
Pessimism is bad for your heart, bad for your immune system, negatively influences your relationships, and impairs your ability to be effective in your work. Oh yeah, and it is no fun.
But you're not pessimistic, right? Compared to the "average person," you are probably less pessimistic than most. But pessimism is not on or off —
it is a graduated scale from extremely pessimistic to extremely optimistic, and all of us are somewhere between those two extremes.
That means that no matter how optimistic you may be, you can improve your thoughts and become less pessimistic, and if you do, you will improve your health, your relationships, and your ability to achieve.