Spend some time deciding if decisiveness is a virtue. Dwell on it until you've convinced yourself that decisiveness is good, and have come to terms that you are not decisive. Around here it may be tempting to label decisiveness as rash and to rationalize your behavior, or not worth the work of changing, if so return to step one and reaffirm that you think it is good to be decisive. Now step outside your comfort zone and practice being decisive, practice at the restaurant, at work, doing chores. Have reminders to practice, set your desktop or phone background to "Be Decisive" in plain text (or whatever suits your esthetic tastes). Pick a role model who takes decisive action. Now after following these steps, you have practiced making decisions and following through on them, you have decided that to make a choice and not dwelling on it is a virtue, Now you can update your image of yourself as a decisive person. From there it should be self sustaining.