So when is enough, enough?

So, one must start to wonder when a friendship gets to the point where they must say enough is enough. Ever had one of those friends that you just give and give and get nothing in return? I had a friend turn out as such, she was so great for such a long time then whether she went crazy or I just found out who she really was is debatable. I seem to blame myself for the braking down of the friendship. What did I do? How could I have helped her more? Was I a good friend? All these things run though my mind as if I could have changed something. I spent a lot of sleepless nights trying to figure out all the answers to these questions. I had a lot of close friends that tried to be unbiased but unfortunately to no avail, because they had already seen the light. How do you go on knowing that you have done everything right(or so you think)but that it doesn't work out in your favor. Or now staying up wondering if they even cared about you at all because they don't seem at all upset about the now lack of friendship. In some respect I feel like there is an unspoken vow that you take as someone's friend. Even though they may suck at being your friend you know that they need you. I just want my old friend back without the new bullshit. She hasn't been the greatest friend in recent months but if I called her could we fix it? Could she still be in there somewhere?