You suffer from 'Obsessive Love'. You believe the myth that out of 7 billion people on Earth that she is the only one who can give you what you need. And, the fact that you can't have her makes you want her even more. It is very difficult to talk yourself out of your feelings but you have got to try. You have to stop romanticizing your feelings for this girl and see them for what they are; a mental disorder. Furthermore, there is nothing that makes a man LESS attractive to the woman he is pining for then the behaviors you can't help but display when you are unequally enamoured with a woman.
Jealousy, insecurity, neediness, desparation, and klinginess are the most unattractive traits you can show a woman. They all communicate the subconscious message that she is way more than you deserve to have. Woman are categorically most attracted to the men that send them the subconscious message that they can be easily replaced. Unfortunately, this is not something that can be faked. Women are far more intuitive than we are and can pick up on the most subtle ques that we give off. Therefore, you must first make yourself believe that she can be easily replaced. Furthermore, you must make yourself believe that you can never be the most attractive man you are capable of being while in her presence. Regardless of whether or not your ultimate goal is to have her back or to put her behind you, your course of action MUST be the same. You must get over her and put all of your energy into becoming the most attractive man that you are capable of being. Go to the gym, get a tan, immerse yourself in the pursuit of the most lofty professional goals that you can possibly achieve, find a new and appealing hobby or sport, put together the boldest and most sylish wardrobe you can afford (women are more attracted to men that LOOK good than they are to good-looking men, whereas we are most attracted to the best looking women regarless of the presentation or lack thereof). In regards to her, you need to stay away from her and initiate no communication with her. If she approaches you, or calls you, that is GOOD. Be polite, and seem happy to hear from her, but communicate the SUB-conscious message that you are a busy person and have a limited amount of time to give. Be subtle. If she asks if your busy, or if you have time to do something with her, you only need to give a second of hesitation before agreeing in order for her to get the message. The best thing would be to make her change her plans to accomodate your new and extremely busy life.
If you do these things for the right reason (because you love youself most of all) you will simultaneously become more attaractive to her (and every other woman in the world), and you will gain the feeling of self-worth to realize that maybe you can do better than her; maybe you can more fully enjoy the pleasures of life without her; maybe she was holding you back. All of these things are true.
You must send every woman that comes into your life the message that you love YOURSELF most of all. It sounds selfish but this is what you must do to make the woman think that she is getting the best part of the deal.
Some of these views of male/female relationships may seem politically incorrect at best but you must remember that although views of sexual equality have changed, we still have the same DNA that the cavemen had, and women are still hardwired to be attracted to the dominant alpha males that are best equipped to provide a secure nest for them to raise children.