In counselling we use affirmations to help build clients self-efficacy. The transformations people can have if you continue to apply affirmations into therapy and every day life, speaks for itself. It is known that counsellors who utilise affirmations with clients who have addictions have seen positive improvements in both building rapport but also when they achieve goals. It is important to note that affirmations need to be sincere as it can ultimately be seen as disingenuous otherwise, which can cause a break down in relationships and mistrust to surface.
For clients who have low self-esteem the repetition of "I am" affirmations can be profound. In fact I have seen it with past clients, whom from having a negative mindset have flourished and come out of their shells becoming so much more confident and ultimately loving themselves, which is so special to witness.
Quite often, these negative mindsets people have, have been present for quite some time and it is common that their own loved ones and/or confidants were the people they trusted implanting them in their minds. Thus, it takes a lot longer for them to rid their minds of such maladaptive ways of thinking. The process of affirmations requires a lot of repetition and practice, as we are teaching the unconscious and subconscious parts of the brain to dispatch optimistic and hopeful communications to our conscious brain.
A helpful tip is if the client or person who needs to elevate their self-worth stands in the mirror and repeats the "I am" phrases as they look at themselves. Also, writing it on the mirror and sticking notes on places you always walk by helps to reiterate these sentiments.
Needless to say, everything takes time, however the fruits of your labour are spectacular!