We study the situation.
It does not matter how long they have met and how long they have left. The degree of influence of the former girl on your beloved is important. Let's start with the appearance. Before meeting with her, he listened to rock, walked in black and grew his hair. And her older brother and all the friends were football fans - that means they shaved almost nalyso and wore narrow jeans. And you got it already with a full wardrobe jeans and out of habit does not miss a single football match, although previously did not even know the rules of this game.
On the second place - hobbies: favorite music, cinema. Suddenly, you realize that all the songs he advises you to listen to are somehow connected with it. You want so much that you also had your own music, but the place is already taken! The former girl in her time so clearly defined what romantic melodies are, that he does not even want to hear about the new bands.
And with the phrase "We'll definitely watch this film, I've already seen it, I remember, then I really liked it", the desire to even think about the movie will be lost, because you know exactly with whom he looked it before. But the most terrible thing is that her influence begins to spread to you! Liked begins to give you advice on how to dress, paint, style your hair.
We begin to act.
In all respects the ideal option - do not know anything about his ex-boyfriend. Believe me, it will be calmer. If you are not familiar, and the guy does not even talk about it - the most stupid thing you can do is try to find out everything about her. Even if curiosity does not bother at all, it's better to cope with yourself than ask the guy about his past relationship. So you yourself will make your loved one think about your ex-girlfriend and launch a terrible mechanism of pleasant memories and comparisons. Believe me, it's much easier to fight with curiosity than with bouts of jealousy.
If you are not so lucky, and you are familiar with the former girlfriend of the guy or the guy talked so much about her that you remember even how her hamster's name is, then you will have to seriously fight the ex-influence. Take all his thoughts: the general plans and memories should completely "erase" the former girl from his memory. If time goes on, and the guy continues to behave as if they broke up only yesterday, try to understand why he can not forget about her, and eliminate the cause, not the consequence.