If you proclaim what you can do, you will do it, even if you put a negative twist on what you are going to do. I said I would be a successful teenager entrepreneur. I make hundreds of dollars a month and am now publishing eBooks on Kindle. I said it would be hard for me to reach under 5 minutes in the mile at the beginning of the season, and it has been hard ever since (I now think it will be easier for me to achieve this goal). I have rejected some of my ideas and never continued them while I have finished some of the ideas that I was previously working on.
What you say about yourself can be either very motivating or can sink your dreams.