You don't always have to be so optimistic about everything because there's actually a lot of power in negative thinking. This video explains why being too optimistic about your future goals might actually hamper them instead of help them.
This video, from the New York Magazine's YouTube channel, suggests that a little bit of pessimism is good for you. In fact, Psychologist Gabrielle Oettingen suggests you might even be better off being a little pessimistic as opposed to being positive about everything.
When you indulge in positive fantasies about completing your goals, you can fail to push yourself the amount necessary to actually do what you're trying to do. You end up imagining how great it will be when you're done, but you never actually do the little things that get you there. So embrace your negative thoughts, and maybe even use them to plan ahead. Daydreaming is fine, but embracing your negative thoughts will get you moving.
Pessimism Is Good for You [YouTube]