I don't see how this works, or at least how it could work over a reasonable period of time.
The reason for repeating these sentences is to alter your beliefs, right? I don't see that it could be so easy to kid yourself into believing something.
Whenever I tried affirmations, I felt wrong - I didn't believe what I was saying, and I actually felt a bit silly, like I was trying to fool myself. Does anyone else feel that way?
The only way I have found to alter my beliefs, is to confront the negative belief; to take time out to rationalise why I should change my belief to one which will help, rather than hinder me.
It's a very liberating experience - it gives a sense of self-empowerment -and it saves a hell of a lot of time.
If affirmations work for you, that's great, but if like me it just doesn't work, try talking to yourself about the issue, as though you were helping someone else with their problem.
Start from the perspective that all limiting beliefs are wrong - we are all capable of so much more than we realise. Debate with yourself how you came to get the limiting beliefs, and why it is wrong that you have been holding onto them - I have found I can change my beliefs sometimes in minutes, and I get a kick out of it when it happens.