1. You feel guilty - was it my fault?
If you were with someone who was a complete tool and make you feel that everything you did was wrong, then you're probably feeling like the breakup was your fault. Were you never good enough? Were you a supportive partner? It is these doubtful thoughts that have you in this never ending guilt trip.
2. You never got closure
Perhaps the guy/girl you've been seeing has moved away and your relationship became long distance. Although you have grown apart, and things may have been rough, when he/she broke up with you, you never got closure. Closure means discussing things and understanding why they don't work. Perhaps you just wanted honesty and your ex partner went MIA. You keep rehashing the relationship and the breakup over and over in your head and don't know what to think. This is hard and the best thing to do is recognize that, if something was meant to be then it would have been.
3. You miss having someone there
Not necessarily that you miss that partner but perhaps you miss having someone there to eat with, cuddle with etc. The qualities of having someone there versus missing the actual person may be the reason you are focusing on your ex versus your need to be with someone.