To answer the second question in the title of this thread, now that people have answered the first one and it isn't helping:
No, jones. I don't think you can move past this. I think you are stuck there, you do not have any choices except to wait for G to decide what he wants and if he decides to be with J, then you should be quiet and make it as easy for him as possible. If he wants you to be there while they hang out or have sex, then you should because it will prove that you love him more than she does. Maybe he will change his mind and decide he loves you more than her after all, or at least ALMOST as much. Just make sure you do whatever you can to keep G happy, otherwise you will NEVER move past this.