Here is something I learned about affirmations. If the affirmation is too far from your actual beliefs about yourself, you will just fight it, and it won't work. Choose affirmations that are an improvement for you, that will help you, but that are not too big of a step for you to start to believe them.

If you need to work on an affirmation, and it's a hard one for you, tell it to yourself in the second person ("You are a worthwhile person with many wonderful qualities.") Also try writing it on a note or sign and putting it where you will read it frequently, or asking someone else to repeat it to you. When it starts to get easier, then you can tell yourself more directly, and start to use "I" instead of "you."