Lola makes an excellent point about introversion/extraversion. I've been through the whole Myers-Briggs thing - basically it's a personality assessment and categorizes four distinct parts of one's personality - which confirmed that I'm a strong introvert. That doesn't mean, of course, that I can't go and mingle with people or speak in front of a group (which I do on a regular basis as part of my work) but it does mean that it takes a lot of energy for me to be able to do that. I very much enjoy it, but once my stage time is over, I need alone time to rest & recuperate.
Seems your girlfriend gets her energy from being with people, while you get recharged from being on your own. It's not that one way is "better" than the other, it's just different.
Now here's the crux, though: It's up to the communicator to make sure his/her message is properly received. If your gf just isn't "getting it" - in the sense that she really can't understand why you are the way you are - then that's because you're not speaking in a language or in a way she understands. Yes, direct is good, but of course you have to be speaking in a way that makes sense to her. So find out, through observation, whether she's a visual, auditory or kinesthetic learner and use that information to express your needs to her in that way. (Learning styles are a whole different topic, but for a brief overview, check out this:
Learnng Styles take your test.)
Good luck!