While I fully understand your sense of responsibility, and your desire to help this girl, it will not benefit her, long-term, if you stay with her, 'romantically', against your will.
I am really not sure what is the best thing to do, because you say that, apart from you, she is alone, and she is obviously very vulnerable. You want to do the right thing ~ for both of you.
A "fait accompli" is a done deed. So, if you just present yourself to her as a friend, with a girlfriend, then she won't get the wrong impression. However, she may become very hurt, and you do not want to push her beyond what she is capable of coping with.
One thought ~ Is it possible for her to stay with the friend she intended to share with over the holiday?
She has a job and an apartment, so she is coping. Now she needs to make some new friends. Perhaps you could suggest some ideas to her ~ clubs or societies perhaps. There may be something at the library, or a sports venue, etc. Amateur dramatics may be a good idea, or a writers society. Trying something completely new might be a good idea.
You are currently apart, but friends. That seems like a good place to be. You can support her, without giving her false hope. Make it clear that you are definitely there for her, but that dating seems to keep ending negatively ~ in break-ups, so you think that there is a much better future, for you both, if you remain just good friends. See the positive in it ~ otherwise you could end up resenting her ~ which would be bad for both of you.
I am guessing that, in a situation where she is alone, and has always felt unwanted, (you feel that) your rejection could push her over the edge. I do appreciate the potential seriousness of this, so I really think that counselling should be the way to go. Could you take her? Could you
see a counsellor, yourself, just to get some advice?
I wish that I could be more helpful, but she probably needs expert help and support, to enable her to cope with things as they are. I agree with BLR that she really could do with the expert help of a counsellor and the support and friendship of someone who cares about her.