Negative thoughts can be imprinted as a habit we would like to shake free from.  
There are steps one can take to start to free oneself from the patterns we have developed.  
The first and biggest step in in simply recognizing you are having them, noticing that they are there.  Over time, by noticing and naming them , we see them for what they really are:  a habit.  You can shift your mental playing field with practice.