These words come from a French pharmacist turned psychologist, Émile Coué de la Châtaigneraie. According to him, by repeating this statement every day, you subconsciously influence behavior and produce positive change. I believe there is some power in believing in the positive, especially for the Christian. I'd go as far as to say this is Biblical, and commanded. It doesn't come from the rote repetition of words, but from how we practice sanctification. Sanctification is where our repentance turns to positivity, when the meaning behind our action turns from "in order to..." to "because of". We read this in the Bible:
2 Peter 1:3-11
3 His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to[a] his own glory and excellence,[b] 4 by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire. 5 For this very reason, make every effort to supplement your faith with virtue,[c] and virtue with knowledge, 6 and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, 7 and godliness with brotherly affection, and brotherly affection with love. 8 For if these qualities[d] are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. 9 For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. 10 Therefore, brothers,[e] be all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall. 11 For in this way there will be richly provided for you an entrance into the eternal kingdom of our Lord and Savior Jesus Christ.
2 Peter 1:3 Or by
2 Peter 1:3 Or virtue
2 Peter 1:5 Or excellence; twice in this verse
2 Peter 1:10 Or brothers and sisters. In New Testament usage, depending on the context, the plural Greek word adelphoi (translated “brothers”) may refer either to brothers or to brothers and sisters