Why you should not stay friends with your ex.

Talking to your ex could be a touchy topic if you are in a relationship.
Contacting an ex could ignite jealousy in a not so healthy relationship.
There are some people who are comfortable with their mates having a relationship
with their ex.

My girlfriend is still friends with her ex, with whom she had quite an intimate relationship, emotionally as well as physically. How do I cope with this?

“She loves him somewhere down in her heart”.
What do expect to do? Pry that out? Pardon me, but you can’t decide what someone feels for another. Even if the “someone” is your girlfriend, and the “another” is her ex. If she shared a special understanding with him, she’s trying to hold on to that. The “ex” in question; could have also been a friend.

If I were in your shoes, I’d tell her, that I only expect her to be more honest with me, and that I’m open and willing to understand her problems. Yes the secrecy could be a colossal bitch later.

If you keep poking her, questioning her; you’re only going to antagonize yourself. You’ll come of as bitter, possessive and obsessive, and soon you’ll be her least favorite person.

Alternatively, if you can’t put up with this, you can confront her, and expect her to make things right. But, the smart bet won’t be on you. Or you could work on becoming the one she wants to talk to.
And pardon me for this. But. Frequency of calls? Duration? Second Favourite?
Strap on a pair dude.
Women do this. Believe me; they do. More often than you think you would. The reason being either of “I don’t know why” or “you won’t understand”.
Be a man. Be cool.Walk away with pride if things get out of hand. And with your family jewels. Intact.

Source: Quora