So angelofdeath, you are a dude actually. Didn't you post a similar question which was merged with this thread.
Past Vs. Present
https://www.istorya.net/forums/index....,132168.0.html
Anyway, dude, it's not that your lady love is stuck in her past no no no it's just that she is still in love with her ex-. Though I am sure she has feelings for you but that doesn't mean its as strong as the feelings she had for her ex-.
Because IT IS AS YOU SAID, if she does love you she should be with you. But where is she now?
All I suggest dude is if you really love the girl truly really deeply then let her be happy.
If you try to do something silly like proving your love, fighting for your love, you'll only be a hassle and a nuisance to her and for yourself. Bear it like a man with a measure of dignity. You haven't lost, it's just that she chose her happiness. You're not at fault, it's just really how Love and Life works.
She is not the girl for you because simply if she is, then she would have stayed with you.