I agree with what Ripose said above.
It's good to use affirmations, but they do need to also be realistic.
Otherwise, when you recite them you'll feel as though you're lying to yourself...which is never a good feeling! I've been using positive affirmations to raise my self-esteem, and I've had this happen a lot of times. I found that when I said an affirmation, I couldn't really bring myself to believe it...which defeated the whole purpose of saying affirmations in the first place! I finally had to re-write some of them to align them with reality. Since then, I've had much better results.

And also: you probably know this already, but I'm going to suggest it anyway because it's been a huge factor in my own experience with affirmations. When you say an affirmation, make sure it is always an "I am" statement, not an "I am not" statement. For example, it's better to say "I am loveable" than it is to say "I am not unworthy of love". To be honest, I can't really explain why it works this way...but I do know that I've had a lot more success with "I am" statements, rather than "I am not" statements.
Anyway, hope this helped you!

Good luck!