One problem I see with self-affirmations is that they only change your perception of what you can achieve, not what you actually can achieve. No amount of self-affirmation will help you become a painter unless you take the pain of learning how to paint. And failed expectations after some time build frustration. So you must be cautious of what you make yourself believe. There is a joke about a guy that was praying his whole life asking god to let him win a lottery. He never did, and after his death he asked god why. The god answered "What could I do? You never bought a lottery ticket".