Hey there, you have come to the right place to find friends and support.
First of all, can you live with this girlfriend and I am wondering why you live together off and on. Who's apartment or home are your living in, yours?
You have shared your feelings with her and it feels like she is giving you a trite answer.
There are some things you can do to decide if you are in the wrong relationship and if you need to get out.
Are you happy in the relationship? Sit back for a moment and think to yourself 'Am I happy in my relationship?' If you are happy, then great! If you're not happy then you need to ask another question. Ask yourself 'Can I foresee myself ever being happy in this relationship?'
I want to mention here that the small changes must come from inside you. You do not have the power to change anyone else (including your girl friend.)
You also need to look at every possible aspect of your relationship with your girlfriend and see if the good outweighs the bad, or if the bad outweighs the good. Sometimes your judgment is clouded when you only focus on the good or only on the bad. The bottom line is to do what you need to do to have a more fulfilling and happy life.
I get the feeling you are afraid to be alone and you are grasping at any time she will spend with you as a good thing and when she is not there you feel really depressed.
Remember only you can make you happy. Take care of you and keep posting.