Optimists tend to interpret their troubles as transient, controllable, and specific to one situation. Pessimists, on the other hand, believe that their troubles last forever, undermine everything they do, and are uncontrollable.
To me, fearless is not the absence of fear. It's not being completely unafraid. To me, fearless is having fears. Fearless is having doubts. Lots of them. To me, fearless is living in spite of those things that scare you to death.