Okay, so I'm not saying that affirmations never work, because under certain circumstances, they do. But if you're inclined to state things in the positive in hopes that this alone will make them come true, you might be wasting your time. Let's look at why.
The human psyche is an incredibly complicated computer that isn't particularly responsive to being reprogrammed, and will "undo" new commands fairly quickly. I won't get into why we are so complicated, except to point to our super-emotional nature. (There are other reasons, too, but this one's a biggy.)
Anyway, we build our personal belief systems very early in life, based on our family's and culture's beliefs, our early life experiences, and our childhood choices. Some of these beliefs (such as, "I am stupid," "I am poor," and "people can't be trusted,") are formed before we even have words to describe them. And most of our individual worldview is firmly in place by the time we start school.
Because of the deeply entrenched nature of our beliefs, it is difficult to change them, although it can be done over time or by using specific belief-changing techniques such as Byron Katie's "The Work," Emotional Freedom Technique (EFT), the Belief Closet process by Lion Goodman, Neuro Linguistic Programming (NLP), the Sedona Method, etc. These processes work by bringing limiting or negative beliefs up into consciousness and releasing them. (They must be faced---not kept down, through denial). Only then can they be replaced by more desirable beliefs, such as "I am beautiful," "I am prosperous," or "I am in a great relationship."
When we attempt to "dub over" an existing belief with it's opposite, all it does is introduce conflict into the psyche. Instead of our minds saying,"Hey, let's adopt this new, more positive thought," it just says, "Whaaaaat? That's not right," and kicks the new thought out. There's no place for it within the established paradigm. It doesn't fit. It doesn't compute.
On a Positive Note
When do affirmations actually work? Affirmations affect change when they're believable enough and don't cause the psyche to spit them out. For instance, you might look in the mirror and think, "I look okay today," and then decide to upgrade that thought by saying, "You know what? I look good!" Or when you've never hit a home run, but you've visualized it enough times that you start believing it's true. Or when you've done the work of unearthing a negative belief so that a new, better one can be planted. Or when you can feel that something is true even though you don't fully believe it yet ("I deserve to be happy").
If you'd like to change your limiting or negative beliefs into more positive ones, don't rely on affirmations alone. This is like cutting grass, continuing to water it, and hoping it doesn't grow back. Instead, check out some of the great tools available (see above) for getting to the roots of your unsuccessful belief systems and changing them, once and for all.