Thank you for your question. From a jurisprudential perspective, affirmations as a form of self-help are generally not problematic, as long as they are not damaging to one's faith or are such that they attribute partners to Allah. However, Islamic ethics and self-development already have a wide range of slogans similar to affirmations which are not focused on the individual, but rather build the individual through their understanding of their relationship with God. These slogans are in the form of formulaic remembrances such as "there is no god but Allah" and "there is no strategy or power except by Allah." The affirmations suggested by non-Mulsims rely on a humanistic philosophy of individualism which is at odds with Islamic philosophy, which sees the human as powerful through God and powerless in front of Him. Awakening the greatness within is through this relationship, rather than an affirmation of the ego separated from the divine.
May you always be successful