You used to be together and when you were together you developed a natural desire to protect your relationship.
Jealousy is simply the expression of that desire to protect your relationship.
Whenever another guy would come closer to your girlfriend, you would wake up this emotional reaction which would sound like:
"It's with me she is. Stay away from her!"
Yes! This is jealousy!
Ans there is nothing wrong with expressing small dozes of it in your relationship as long as you don't become a slave of it.
Now, what happens when you break up?
You end a relationship but some of the emotional reactions associated with your ex stay in your mind.
You might feel desire for her, slightly possessive, or even imagine your future with her even though you rationally know that you are no longer together.
To stop the jealousy pattern, you need to consciously reprogram your mind and develop a new set of emotions when you are around her.
This whole process + the strategies to create that shift are described in detail in the e-book and MP3 audios available at:
http://vitalcoaching.com/malejealousy.htm
It is actually relatively easy to get rid of unwanted jealousy if you understand the emotional dynamics associated with it.
Most guys keep on going in jealousy responses when it is no longer needed or appropriate.
It doesn't have to be that way.
If you are trapped in that specific pattern (being jealous when you see your ex with another guy) get this material now:
http://vitalcoaching.com/malejealousy.htm
It is the power kick you are looking for
To your power!
vitalcoach