Read LUke 15:11-32.
I am not implying that you should just take her back and marry her but you should not be holding a grudge against her for departing you. You were both young.
Of course, forgiveness is the key here. Once you truly forgive her, only then you will come to know if she will be right for you or not.
Remember, she was honest with you about her feelings back then. Give her credit for that. However, if she is on the rebound, you should be able to see those signs.
A unforgiving heart can blind you from the truth. This is something my wife has come to realize these past few months.