If you have a garden, and you only fertilize the weeds, but not the flowers and the vegetables, then the plants you don't want will grow bigger and stronger than the plants you want.
The same thing is true with your thoughts. Most people have both good thoughts and bad thoughts running through their heads. Unfortunately, it's easy to fall into the bad habit of dwelling on and nurturing the bad thoughts. Like all habits, running a discouraging internal monologue can be a tough habit to break.
What you feed grows. Make the decision to dwell on and nurture postive thoughts about yourself and others. When negative thoughts come to you, put them aside and think about something else that's positive instead. In time, thinking positively will be as much of a habit as thinking negatively used to be.