also weighing in from the female side of the court, i would say she is desperate. now hold your offended feeling for a moment, i'm not saying she is desperate because she wants you, i'm saying based on her reaction to your quick change of mind she is in a desperate situation. now sad and painfull as this is to say (even on the interwebs) i've been where she is. (it was a mega long time a go before any of you were born!!!) and the best thing for her is in this situation the best thing for you as well.
its also the hardest thing.

she needs a friend. she needs someone who can help her pull her self esteem out of the clogged toilet it is slowly choking to death in. now other people will tell you that you can pull it out for her, not the case. only she can do that. but good friends help, friends who actually care weather or not you get up and come to school make all the difference in that kind of dieing world.
BUT
you cant let her pin her hopes and dreams and life on your head. you cant allow her to emotionally black mail you into doing anything EVEN IF IT IS WHAT YOU WANT TO DO.
that's where it gets really really hard, it takes a massively strong person to say "well i wanted to do this for you, and i was going to do this for you, but now that you have emotionally black mailed me into it i cant. because i respect you too much as a person to let you put me in a position where i will loose respect for you. and if you continue with this emotionally black mailing behavior i cant stay with you even as a friend."

you see it has to be black and white that you do want to stay her friend ( you do want that right? i did infer that correctly?) and it also has to be black and white what your problem with the relation ship is. (the problem is that she is emotionally black mailing you by saying "i cant live without you in my life") to continue to be her friend, and a true friend not just a shoulder to cry on and spare change for the buss, you have to be invested in her total well being. and that means NOT enabling this emotional black mailing thing shes got going on.

codependency is a hard habit to break. but with your help, maybe she can.