What am I supposed to do ???

So I'm dating a sorority girl. Something I've meant to do for a long time. We have a lot more in common because we're both in the greek community.

But she's got to be the worst girlfriend ever. I buy a card every month for our anniversaries, even got flowers last time; she doesn't ever get me anything. I always try to schedule cool dates, but all I hear is "maybe" and then she's always too busy. She didn't even invite me to her sorority formal.
Everyone keeps telling me that I should just leave her.


but here's the rub. how can I do so and not come out looking like an asshole in front of her sisters?