When is enough, enough?

I am a 20yr old female in college, I met this girl 3 months ago through an organization I joined and we have lots of mutual friends so we started hanging out a lot and found that we had many similar situations/circumstances so we became quick friends. I also quickly found out that she handles her problems a lot differently than I do. I have always been there to hear her out every time she has done something, well, mindless, to get her mind off her real problems, things which include bulimia, starving herself, binge drinking and throwing herself at guys. She asks my advice, I give it to her, and she still does the exact opposite and then comes right back to me asking for more advice to get her out of the even deeper hole she's stuck in. She called me tonight and I didn't answer because after spending a whole week and almost all of today of hanging out with her, basically counseling her, I am drained and I cannot deal with any more her problems. It's like I'm a Catholic priest and she's coming with her confessions, to lay her burden on me. When I didn't answer she left a voicemail, a text, and then sent me a msg on facebook saying that's she's sorry if she woke me up but she "really needs someone to talk to right now"....I don't feel bad about not answering, but a part of me still wonders if that was the best thing to do. What do you think?