Your description of your situation almost exactly matches my relationship with my BPDgf in college. I, too, spent all my time with my girlfriend and had a hard time establishing boundaries to protect time that I needed for myself. I wanted the relationship and didn't want to threaten it by making her unhappy, and I also didn't want to hurt her.
What I've learned from all the time I've spent on bpdfamily is that so many of us don't set the boundaries we need to set, or look out for our own needs, because we are scared to threaten the relationship. But over the long run, our failure to set boundaries actually threatens us and the relationship more!
Another thing that is important to understand when setting boundaries with a person you feel is vulnerable is that "hurt" is different than "harm." We might tell our girlfriend that tonight we need to sleep alone. This may hurt her, but it will not harm her. We want to be compassionate, but it's not our responsibility to protect everyone around us from discomfort. Does this make sense? This is a very important concept, and I know it's much easier for me to say than it is to put into practice. I have been there!
Ideally, which nights in the week would you like to sleep with her and which nights would you like to sleep alone?
RC