Because the mind works so fast it also has a tendency to disappear into unwanted areas. You can call this wasteful, negative and useless thinking. By allowing your mind to get lost into any of these three will result in a great loss of time, energy and enthusiasm.
Just as if you were driving a very powerful and responsive sports car, you would have to be very alert and attentive about what you did with the controls or you might end up in the ditch, so too the mind needs to be shown the same attention or it will end up in the ditch of negative thinking.
Just as with the example of the sports car it will take a lot to get it out of the ditch so it will take considerable attention to get the mind back on track.
By maintaining a simple discipline of checking from time to time to see what thoughts you are having and if they are supportive or detrimental to your purpose, you will, in time, be able to keep your mind focused and empowered.