How should I talk to my girlfriend about her still liking her ex?

Question: why is she having deep conversations with another guy? I think that's unacceptable. I think you should just flat-out tell her that you want to know what's going on, why she's having these conversations, why she plays online games with him, why she is having a close relationship with another guy. Confront her with all the evidence. If she tries to dodge it, corner her, don't let it go. Make her answer.

Basically give her an ultimatum: it's you or him and you don't want her near him anymore, and be willing to stick by your word.

Don't let her walk all over you.

I don't want you to try to reason with her any further. No need to get into any more debates. Now your actions are going to speak louder than your words. You won't appear controlling or insecure if you make your point non-verbally.

Here's the plan: I want you to stop taking your girlfriend out on the weekends. This will be a strategy of withdrawal. No more weekend dates with her, for now. That includes Friday, Saturday and Sunday. You're only going to see her on a Monday, Tuesday, Wednesday, or Thursday.

If your girlfriend has any brains at all, she'll put two and two together and realize that her selfishness has caused you to withdraw. Then we'll find out how high a priority you really are on her list. If she doesn't change her behavior after this, then she's not really as "serious" about your relationship as you thought.

Remember guys, if you're going to go the distance with her, there can't be any boyfriends in the background.

If she's still having feelings for her ex then she's not ready for a relationship just yet. She's gonna end up hurting you if this issue isn't addressed. Her feelings are causing her to hide things from you and that's not fair to you. Nobody deserves to go through that pain.