I struggled with jealousy for years. I learned to deal with it by understanding its meaning to me and what it is in psychology, sociology, and literature. I don't mean go back to school, just type it in your browser jealousy+psychology for instance. Wikipedia is a good place to start.
Then, I learned what envy was. Then I compared / contrasted the two realizing that I wasn't jealous, I was envious in many cases.
Then I looked at myself. I made a positive list of qualities of me. Or, in a sense defined me, which is on going for every individual, I think. Next, I made friends with myself with the attitude of acceptance.
As far as your ex talking and hanging with other girls. That will happen. Some of my ex's are married with families today and envy creeps in when I encounter them, but I sweep it out with examining 'positive me' real quickly, focus on the 'now,' and then enjoy the interaction with her for that moment.
I don't know if that helps or not. I had two cents and tossed it in.