One definition for insanity is doing the same thing over and over again and expecting different results. But it’s easy to keep repeating the same behaviour. As a whole, I think humans are resistant to change (personal opinion not based on scientific fact or anything).
So for today’s prompt, give us some backstory (yep, dreaded I know, but can be useful) as to what action your character repeats and what outcome they expect. Now put them in a situation where they do the same thing yet again. Or alternately, but them in a scene where they have thought through all the things they usually do and come up with another alternative. What are the results this time?
So if the definition of insanity is expecting different results with the same behaviour, is the definition of pessimism doing the different things and expecting the same results? Apathy: Doing nothing because you expect nothing (yeah, whatever). Optimism: doing different things and expecting different results.