Mommag2 brings up some interesting points. Don't give up! Amalia, perhaps you will have some luck investigating your options in other councils, if you have them at your school. Most of the cultural and multicultural greek organizations are open to people of all ages. I think the only difficulty you might run into with a non-NPC sorority may be the fact that you already have one degree, but I seriously doubt your age would be an issue. With regard to "fitting-in," I think you'd be fine because you can typically find a diverse range of ages, even in undergrad.
Edited to send some LGLO love to Mommag2 & the Gamma Ladies!