I'd say that chances are that if you focused on travelling to mars in back in the next 10 minutes, some element of doubt or queasiness would play into your mind - when doubt is introduced in some form, positive thought is, for all intents and purposes, weakened. The work of AbrahamHicks puts it wonderfully and there's centuries worth of contents to check out from them, check it out and you may be on your way to some super powers sooner rather than later.

Yes.

Kind of. I'm not sure about the cosmic and spiritual aspect of it, but having an optimistic and positive perspective on life inspires me to be productive and work towards my goals. It also makes me more likable to others, since people tend to gravitate towards things that make them happy.
Similarly, a pessimistic attitude would negatively affect those around me and generally make me a less likable person. I also wouldn't want to do anything, because I would keep telling myself "what's the point? It won't work out so it's not worth the battle." And if you keep telling yourself that, you'll never know what you are capable of.
Obviously you're not going to transform into a superhuman or anything, but it's a good idea to focus on the aspects of your life that make you happy and stay positive.