Once source of Imposter syndrome is "not enough." Trying to use tools that help you have more gratitude and see abundance can help. You are enough just as you are, but when you compare you with others or even with the idea of the you you'd like to be or think you were you go Coocoo. Journaling can be a big help, especially if you write without a filter (I.e. Write down all the stuff that comes up, even the stuff you are scared to admit). Writing a list of things you are grateful for is another good exercise - 10 things every morning. It sets your mind to a positive place to start the day. Another good practice is something called intenSati. It is exercise mixed with positive affirmations. None of these things work overnight and some are a bit uncomfortable -- just fake it till you make it. Something good will come of it.
What is the purpose of positive affirmation? It sounds like a tool to desensitize yourself to your failure to succeed. But you make it sound like a good thing - so I'm missing something: what is the purpose?