My GF and I have been together on and off for 6 years.
During high school
We often went to the fraternity... She often stays until 3 or 4 in the morning and has stayed there a couple times
They kinda dated a little while
Im sorry man, but this aint gonna work. Frankly i think that you should stay with your friends in the frat and lose the girl. On again off again highschool sweetheart who spends time with an exboyfriend and stays the night at a frat house? She is just going through the motions now.
Am I overreacting?
No.
Should I be understanding that they are the only friends she has?
Frankly, from your description of things, it does not appear that there are really too many friends around; yours or hers.
Good advice posted above.
Cut your losses now. It's a learning experience. You will be wiser because of it.
Might not be easy, might not feel good now, but often the right and best things to do are not easy and may not feel good at the time.