There's some tips for helping the "very social types" like your girlfriend better understand their more introverted partners in this article.
Caring for Your Introvert
I'm kind of an odd E/I mix. Very social alot of the time but only if it's tempered with regular processing/decompression time, so I know what you mean. If I go long periods with no time to myself I can hit overload very easily.
I set a bit of time aside to be alone everyday and treat it as a mental health necessity. That way it doesn't get too piled up on me.