In a word, yes.
Affirmations are repeated positive statements designed to bring about a desired result. The repetitious aspect is meant to influence and trigger the subconscious mind into positive action.
It's no surprise that on a daily basis we often unconsciously repeat negative statements to ourselves about different situations in our lives. In doing so we often bring about the undesirable circumstance itself.
We might say to ourselves "I can't do this", "I'm not good enough for that" or "this will never work".
Needless to say, our proclamations become self-fulfilling prophesies.
The reverse can also be true.
If we were to turn the negative statements we frequently utter into positive ones, we would bring about more favorable outcomes.
Take Muhammad Ali as an example. Who hasn't heard his famous declaration?
"I am the greatest, I said that even before I knew I was." Muhammad Ali
Is there any question in anyone's mind that Ali's repetition of that phrase helped him achieve the status of being "the greatest"?
Another one of his famous maxims was "I float like a butterfly, sting like a bee". No doubt his opponents shuddered at the mere mention of it.
Another interesting illustration for the effectiveness of affirmations comes from the auto suggestion work of Emile Coué (1857-1926) the French psychologist and pharmacist who introduced the now famous phrase:
As a tool for self-improvement, he recommended that we begin each day by stating it firmly and convincingly.
Coué felt that, as stated in his "Law of Concentrated Attention", whenever attention is concentrated on an idea over and over again, it spontaneously tends to realize itself. In working with his patients he utilized this law to help them build their self-image.
Since then affirmations have also been used in cognitive therapy and found to work quite well. Needless to say, there's no magic to it and there are limitations. Certainly you wouldn't make an affirmation that is unrealistic or makes no sense. Ultimately, they are meant to serve as another helpful tool in furthering your personal development.
As you can see employing positive phrases and assertions in the course of your personal development can serve as a useful and productive tool. Anything you can believe, you can achieve.