But how do you know?
According to an experiment conducted in 1990 by Christopher Dewberry and Susan Richardson your mental state might have an effect on your outlook on the future. Specifically your levels of stress and anxiety have a rather big influence. This can apply to everyday judgements or bigger events, for example what are the chances of you finding a 5 Euro bill on the street or what are your chances of winning the lottery.
In a study conducted in a school in England, which tested students for their optimism, by use of a questionnaire, it was found that the ones currently in between two A level exams had a significantly less positive view of future events than the students who had already completed all their tests a week prior. A more pessimistic estimation of the future might in return lead to more feelings of anxiety, continuing the cycle.
Of course, life will never be without stress and important events and there are even benefits tot he arousal state weg et from this, but there are certain coping techniques, coming from a relatively new field called positive psychology, to better deal with anxiety and reduce the effects. Since optimism has been linked to various different health benefits and a longer lifespan it might be worth a shot to look into methods to reduce anxiety, become more mindful and increase optimism in the long run.
Christopher Dewberry, S. R. (1990). Effect of Anxiety on Optimism. The journal of social psychology, S. 731.738.