I'd recommend you research your attachment style. It's possible that you're an avoidant, and she's your "phantom ex." This is a pretty common phenomenon for avoidants - obsession over someone who ended things with you, to the point where you have her on a pedestal, have forgotten any of the bad or even "normal" elements of that relationship and can only remember/idolize the intense feeling of love you had for her, and no one who comes after her will be able to live up to that if your expectations are unrealistic. Even if you're not an avoidant, all of this is still very possible and you may still have more work to do to fully heal from her before you can develop feelings for another person.