I was watching an interview Oprah did with Joel Osteen. He said, "What ever follows I AM will come looking for you." In other words, when we say, "I am tired," we can expect to be tired. If we say "I am never going to find a better job" or "I am never going to have a satisfying relationship," what do you think you'll be bringing into your life.
In the book The Four Agreements by Don Miguel Ruiz, the First Agreement is to be Impeccable with your word. Mean everything you say and only say things that you mean.
Here is a video that gives you dozens of positive, affirmative I AM statements.