Affirmations are recommended as a way to put yourself in a desired state or circumstance. A somewhat derogatory affirmation is often used, namely, 'fake it till you make it' to achieve a result. My sense is that the 'fake it till you make it' has more legs than affirmations on their own. At least there is some indication of effort.
One of the tenets of achieving happiness is to 'decide' to be happy. If it was that simple, the world would not have much unhappiness. Young children don't decide to be happy, they are naturally so. What will it take to recover that natural state so that the affirmation 'I am happy' feels authentic? [Russell]