What should you do if your girlfriend admitted she still misses her ex?

She's being honest with you and you have to give her that even though it hurts you. You have to sit down by yourself and go through this problem step-by-step. First off when you go with someone the two of you make wonderful memories together before the problems enter into the picture. When things go wrong and one or the other splits up with their mate it leaves the one that is still in love with the other with a feeling of grief. It's a grief of losing someone special in your life and grief has no time limit so I can't tell you she is going to get over this guy in a few weeks, month or even a year. It simply takes patience and time. You should sit down with her and let her know that perhaps the two of you should separate until she has had time to clear her head and see which way she wants her life to go. Let her know in no uncertain terms that you aren't willing to sit around and be second best while she is always thinking about her ex. Absence often does make the heart grow fonder, so with you out of the picture it will force her to start thinking and she may just come to realize you are indeed more important in her life than her ex is. Don't sit around taking this sort of behavior as it's not fair to you. At the present time you are enabling her behavior (even though she is grieving) and you are simply going to have to take the risk that if you separate she'll either come back to you or go back to her boyfriend. Either way you win! ask her if she has considered going back to him...if she would..dump her..if not, work things out