Changing a core belief can be done in two ways, the indirect and the direct way.
You may not be able to get rid of the negative beliefs completely immediately and you don’t have to. Change happens over time. What matters is which are the thoughts you hold in your mind more consistently than the rest. Ultimately it is all a matter of focus.
Keep your focus on what you want instead of what you don’t want. Keep your focus on what is good and less on what is not good. Over time you will get where you want to be, more than where you do not want to be. Over time, your positive beliefs will become more and more real to you and your negative ones will just drop away thereby being automatically eliminated.
If you want to eliminate your beliefs directly, you have to examine the roots of your beliefs. Know your meta beliefs (Beliefs about beliefs). You first have to find out the reason of a belief. And know the reason for the reason of that belief. Go back as far as you can until you get to the core. Look for the essence.
How to Overcome Deep rooted Beliefs » Secrets of Mind and Reality