It's about realizing what we are thinking, recognizing our thought processes and consciously changing them. It's not a one day thing and we are never perfect. We do things unconsciously all the time and the challenge we face is finding out what we are doing that is harming us.

It is well known that the brain changes throughout our lives and it becomes habituated to certain trains of thoughts that are also realized as physical changes in our brain cells. This is called neuroplasticity. So we have to work on our brains, we have to start doing the things we want to do and those cells will become stronger as the ones we don't want degenerate.