Never get jealous when you see your ex with someone else, because our parents taught us to give our used toys to the less fortunate
Leave the things of the past and move forward. If you still get jealous of seeing your past lover with someone else, then maybe you still love that person. There is a reason why the two of you parted ways. That means he or she is better left with someone he or she deserves. You deserve much better. That person who is currently going out with your ex might need your ex badly. So you might as well give them the chance to be happy. Besides, you are the first. So you got the better of him or her. Leave what is left to the poor current lover.