From the BBC:
Canadian researchers found those with low self-esteem actually felt worse after repeating positive statements about themselves.
They said phrases such as "I am a lovable person" only helped people with high self-esteem.
This doesn’t surprise me. Telling yourself things you fundamentally don’t believe is likely in many cases to remind you of what you do believe, which may be along the lines of "Who am I kidding, lots of people don’t like me."
I think some affirmations would be less likely to have this effect. Things that are true would be affirmations of the possibility of change and growth, and although they may stir up some reactivity at first they are fundamentally true and it’s therefore possible for our entire being to organize itself around them. Untrue affirmations ("everybody loves me") can only be believed by people who suppress an awareness of the truth (many of those who have "high self esteem" have low self-awareness. Eventually that’s going to catch up on you.