Optimism makes you less likely to walk away while not actually increasing your chances of success. That means that being overly optimistic will make you stick to things longer that aren’t worthwhile. Better to be well calibrated. Life’s too short to spend your time on opportunities that are no longer worthwhile.
Optimistic really ought to be a verb
It's something you *do*, not some property you happen to possess. Sometimes it's easy to do, and sometimes it's darned hard, requiring imagination & determination