In her words, we couldn't be together before we learn how to be apart.
I've been there, and what you are experiencing is perfectly normal. It's the pain of growing apart from her.
From my point of view you made a terrible mistake, which was
She became the most vital part of my life.
It's good to love someone, but no one should be your reason for living, or it happens exactly what happened to you this time. You lose that someone, and feel hopeless and apathetic.
Because she was so important you grew insecure and jelous, and the relationship reached a point, where she or both of you felt it would be best to be apart.
What I say is this: you have to learn to live your life for yourself. Set goals for yourself in work, sports, hobbies. And make an effort to work on them, and put your mind in your actions. It will feel like hell for a time, barren and sterile, it will pain you tremendously, but you have to built a life for yourself, where you are the center of your life, and not someone else. Because otherwise when the center of your life is stolen, you reach this same crossroad you are in.
In my view relationships, they should be attempt by balanced human beings that despite loving the other, put the common good of both first. A relationship should be an added blessing to one's life, not the one's center of gravitation. This enables one to make the best decisions for both, even with hesitation, and find the best outcome for everyone.
Otherwise if you make of maintaining the relationship your central objective, or of a woman the center of your life, you will grow clingy, jealous, and miserable, because you don't value yourself enough, and value your partner too much. You will have very low self esteem and will do things beneath you, putting yourself always in second place. And that is a very unhealthy and unbalanced relationship, and very unstable, and will either end or will continue in an unhealthy and sick manner.
So my advice, is for you to change your goals, make a life for yourself, with goals, and the drive to accomplish them, and when you can balance your life with a girlfriend, engage in a relationship with her, or with someone else.