Look at this realistically. What could you have possibly done to make things different? Nothing. You could have tried to break up the relationship, but that would of just made her want him more. That is human nature. Can I ask, how did she die? She made her own choices. You can't control everything in life and the sooner you accept that, the easier it is to live. There are things that are just out of our control. You are going to miss her dearly and I think that is the main thing going on here. You want her back, and that is only natural. You loved her with all of your heart. And it is very unfortunate that she is gone. But she is, and you have to live on. You have fond memories of you and her together. Treasure that. Accept what has happened. Get some grief counseling to help you though it. There are others going through losses such as yours that you could connect with. It really does help you get through it. But know that there is no time limit on grieving and we don't know how long the stage will last. But anything is normal for grief. It sounds like you need closure. Know we are here to help you with this. But you need to drop the guilt trip first of all. And move on from there.
Hugs, Karen
PS I just read your first post over, and you said you are already in grief counseling. Sweetie, it takes time, so just go with the grief counseling and you will be better. I wish you the very best...