ANSWER: By themselves, affirmations really don't do anything. They are not "magical spells" that cause things to happen. In fact, the verbal part of a spell has little importance.
An affirmation affirms or asserts that something exists or is true. That's all. Because of that, it also puts your attention on what you are affirming, and helps take your attention off the opposite. Repeating an affirmation, therefore, helps keep your mind on what you want, and not on what you don't want. Do that enough and you establish a habit of thought. Choose words for an affirmation that trigger positive mental and physical associations, and you establish mental, emotional, and physical habits. So, affirmations really don't work... but they do.