So true. I also read somewhere that brain does not want you to be happy. It wants pro actively help you survive with fight or flight mode. But as evolved human beings we can program our neo cortex by consciously replacing with positive thoughts when we catch ourselves thinking negative.
It is like you have a glass filled red liquid comprising of negative red thoughts. You pour out some red liquid. Then add pure water ( positive thoughts). The red color will fade away.
Through affirmations and conscious programming we can train ourselves to choose positive thoughts