It seems that your girlfriend might be confused as to her own life and her own goals. I think she needs time to find herself. Unfortunately, if you truly love her, the only thing you can do is respect her need for space. It is hard especially at that age to find yourself and your goals in life with a relationship taking up a lot of her emotional time.
I know it really sucks, I was in a similar situation where my boyfriend wanted "space" to get to know himself better. After days of crying and feeling rejected, I decided that my love for him was strong enough that I could let him do what he needed to do to be a happier person. And at the same time, I took that time out to do things for myself like pick up a hobby or reconnect with long-lost friends.
In my situation, the time apart made my boyfriend realize that he missed me and he needed my support in order to accomplish his goals. And we eventually reconnected and got back together. It is possible that your girlfriend needs some time to reflect on your relationship, and to realize how much you care for her. Considering that you have only dated for 6 months, and you have separated briefly at least twice, the best thing is truly to just let her have her space. It could be that you met her at an awkward point in her life. But have hope, it sounds cliche, but if you two are meant to be, you will get back together. Hope this helps!