Its SOOO hard. I have had to change a lot of core beliefs. I have found that with some of them, I have just had to make the decision to change it. As in, "I am not going to believe X anymore... I am going to decide to believe Y instead." When X pops up in my mind, I tell myself, "I don't believe that anymore-- I believe Y instead."

It takes a while... but it does work for some things.