The use of positive affirmations has been a source of strength and renewal to many people.  Affirmations discipline the mind into a chosen channel and become ingrained into the memory and eventually the character.  You may constantly tell yourself that you are no good and will never achieve anything worthwhile.  When you started telling yourself this, it was not true.  But by believing in your inability to do anything worthwhile, you achieve nothing.  If you think that you are a nobody, you will act like a nobody.  What you believed, you became. 

Affirmation, to be constructive, must always be positive and inspiring.  This does not mean telling yourself something you know is untrue, but it does imply taking what meager talents you possess and developing them to the maximum of their potential with a confidant spirit and a positive faith in the goodness and purposes of God.