I need to have some more personal space.
Is your girlfriend causing you to feel this way?
Yes, she doesn't seem to understand that I need some personal space
I'm sorry she's being like that.  It's totally okay for you to want some personal space.  She needs to respect that!
I know, I'll just tell her to leave. I'm tired of not having space to myself.
I can understand you needing some space. It's OK to need to be away from other people. Instead of sending her away, maybe you can communicate to her how you feel.