If you are asking this question, and recognize that your way of thinking is problematic, that is more than half the battle. The factors could contribute to this way of thinking would be: family history, life experiences and interpretation of those experiences, current peer group, and your own internal wiring and outlook. The important consideration is that you are motivated to do something about it. Recognizing that it is a habit is positive. The focus now is on breaking that habit.

I would suggest writing down your thoughts on a daily basis. This may take some effort but will help you identify patterns to your thinking. Look them over at the end of the day and evaluate how well you did. this allows you to better monitor your thoughts and take control over them. I would also encourage evaluate your environment. Exposure to more positive and diverse situations may allow better monitoring and filtering of these thoughts.