What exactly went wrong? Any Ideas? :/?

I met this girl Senior Yr we became really close great friends, pretty soon she stopped coming to school, stopped talking to me, began using me, accused me of loving her in more than one way, (which makes me sick, b/c I don't roll that way, I'm just a good friend, guess you could say I care to much?) and pretty much said she wanted nothing to do with me... She's the one that used me, she's the one that is ignoring me, she's the one who claims she doesn't have time to talk our problems out... I've helped out with everything gave her gas $, did her school work, was there for her to talk to, to cry on, to lean on, always stood up for her, made sure she was ok at all times... My Friends are my everything and I hold strong bonds with people I like feeling that I'm needed, that I'm loved, that I'm trusted.. But I really haven't done anything to this girl, she just stopped everything all together, and now I especially feel horrible, b/c my friends are treating her badly b/c of the way she's been to me, and she's really hurt me, I'm not over our friendship yet, I still want to know what I did wrong... I can't seem to get an answer though, I have been super depressed, have even thought about Suicide (it's really unlike me) But I can't help but be bothered by the whole situation b/c I try to be the bestest/truest friend that I can be I'm always there for people, But I hate being cool one minute and then someone ignorning me the next for gosh knows what... Any Advice for me? I just can't seem to get my mind off the situation..