If you have just broken up with your ex you are likely feeling very emotional, heart broken and possibly even depressed.
If you spend your every waking hour thinking about your ex, what went wrong with the relationship and possibly how to get back with your ex, you will be stopping yourself from moving on and even eventually making yourself clinically depressed.
What you are feeling is natural, a relationship break up is hard and things will be tough for you for a while. How long this pain lasts will depend on the strategies you put in place to cope with the break up.