In “Positive Self-Statements Power for Some, Peril for Others,” authors Joanne V. Wood, W.Q. Elaine Perunovic, and John W. Lee write: 

When people with low self-esteem repeated the statement, ‘‘I’m a lovable person’’ (Study 2), or focused on ways in which this statement was true of them (Study 3), neither their feelings about themselves nor their moods improved—they got worse. Positive self-statements seemed to provide a boost only to people with high self-esteem—those who ordinarily feel good about themselves already—and that boost was small.

In Why Has Nobody Told Me This Before?, Julie Smith suggests why this is the case:

If you say out loud that you are strong and lovable, but you don’t believe that you are, then your inner critic will get to work coming up with all the reasons why you are not strong or lovable. The result is an internal battle, and plenty of time to focus on all the narratives that bring you down while you are desperately trying to push them away.

Affirmations that are too inconsistent with a person’s view of themselves cause cognitive dissonance and alarm the inner critic. Whether it’s a lack of evidence, or an inaccurate appraisal (and subsequent prediction), it’s too much for a brain to look past.

Unfortunately, we live in an age where positive thinking becomes borderline conflated with, or confused for, willful delusion or reality distortion. 

Instead, we’re better off considering our own latitudes of acceptance: our zones of flexibility with what’s acceptable or not, including our own opinions of ourselves and evidence to show for it.

Smith writes, referencing Wood et. al’s study:

The study I previously mentioned found that when those with low self-esteem were told it is OK to experience negative thoughts, their mood improved. They no longer had to battle with trying to convince themselves of something they didn’t yet believe. Therefore, on the days when we don’t feel strong, we don’t need to tell ourselves that we are. We can acknowledge that feeling this way sometimes is a part of being human. We can respond to it with compassion and encouragement. We can then turn to the things that help us to feel confident in our own strength again, by using all the tools at our disposal to move through tough times in line with the person we want to be. The way that we start to believe something more positive about ourselves is to use action to create evidence for it. 

While affirmations may not be the best strategy for those with low self-esteem, words still matter. If mistakes and failure lead to an onslaught of self-attack, do not let any of it go unchecked. Professional athletes have professional coaches for a reason. In day-to-day life we don’t have that, so we must be that voice for ourselves.