I went through this recently with my gf because she couldn't stop herself from maintaining contact with her ex. Her ex still had feelings for her and she admitted to me that she told her ex that she still missed him (all of this was happening while she was dating me). It tore me apart and it made me feel insecure. I was you. But here's what I did. I realized that she was never going to move any closer to getting over him if she kept in contact with him. Even though I was hurt and super pissed, I talked with her about it (as level-headed as I could manage) and made it clear to her, in no uncertain terms, that I could not be in the relationship with her when her ex is still in the picture. She said I was being controlling and we had our biggest fight over it. But now the decision was hers to make. I know it sounds like an ultimatum, but there's no other way about it when your SO is doing something that's completely disrespectful of the relationship and hurting you. She eventually chose me and promised me that she would go no contact with her ex. Even though what she did hurt me, I feel very lucky that she did choose me and we're working on moving past this. If she hadn't of chosen me, I was prepared to break up with her even though I am very much in love with her. Will she keep that promise to stay no contact? I don't know, but I am making the choice to stay with her and I'm trying to trust her. If she doesn't keep the promise and I find out later, then that would be another reason to break up with her.

Throughout this whole ordeal, I've reflected on three things. The first is that yes, it is a struggle for her. Her feelings for her ex are still there even though she has feelings for you. You need to understand that she has a past with this guy and that neither you nor her can change that no matter how much you wish it. Either you accept who she is and how she feels (given that you can accept that she says that she wants to move on and be with you) or you break up. The second is that while she does have those feelings, it is UNREASONABLE of her to act upon them if she's expecting a monogamous relationship with you. That includes staying in contact with her ex when there's obvious residual feelings between them. You are completely legitimate being nervous and insecure about her behavior. You need to put your foot down on this or the emotional turmoil will continue eating you alive. This kind of behavior is now a huge red flag for me. The third is that she may not realize what she's doing is really wrong or how much it's hurting you. It was the same with my gf. She also said the talking was nothing emotional even though HELLO "I still miss you" was part of the conversation. And she didn't realize it was hurting me until I made it clear to her that it was hurting me enough that I was willing to break up over it. You may have to do the same.

So where are we now? She has admitted what she did was wrong and she has told me she is sorry for hurting me. That meant a lot to me and emotionally I really needed it. She has also told me that making the promise to go no contact has already helped her move a little bit forward. I feel like it has too. She does still tell me from time to time that she misses her ex. I support her as best as I can. What will the future bring? I don't know, but I feel better about it.

tldr: Your gf is not going to move forward if she is continuing to maintain contact with her ex. Put your foot down.