We weren't best friends, but we had a very very close group from middle school. The rest of us were quite privileged compared to her: her dad was straight abusive and her mom made very little selling food on the street. Through high school we were always supportive of her; listening to her issues, letting her & her mom stay at our place when the abuse occurred. She was also a very caring and smart person, but in retrospect, I think she might have had some serious mental issues/damages than we youngsters could have known of.
Then most of us went abroad for college, or went to different universities in the country. We met new friends, got involved in other activities, had issues of our own. We still tried to spend time with her because we understood how much she needed us (we loved her, but did not "need" her the same way). Then she started to complain how the rest of us had changed, made new "close friends" and compared how much they cared for her and listened to her better. I began to feel irritated but my mom told me to try to put myself in her shoes.
And I did. But the last straw was not personal. It was a professional issue. That summer, I organized a summer camp for underprivileged kids and she was a volunteer mentor. The night before, I made the mistake of commenting on her dress (it was a little revealing in the back - that's it!) and she all of a sudden flipped in front of all our friends. She stood up, left and then texted me that she wouldn't go tomorrow. Now I could pass a friend's fight, but I poured my heart into this project and each child was paired with a mentor. Her excuse for the outrage was "I wasn't in a good mood cos my dad hit and pushed my mom into a desk that afternoon". I told her to fuck off cos she was IN A BAR with us the evening of the same day.
After a few months after that, I found out that the rest of our group stopped hanging/talking to her for their own reasons. We don't even discuss about her anymore.