This disease is in its way as bad for the loved ones of the sufferer, as it is for the sufferer themselves.
All you can do is be there for her. That means being ready and willing to listen when she is able to talk - and more importantly, understanding when she is unable.
Depression makes you hate yourself - deeply, and because you feel no sewlf-worth, you figure that everyone else should hate you as much, so you invest considerable time and effort in making yourself as hateful as possible.
So, when she is utterly cruel and hurtful to you, ignore it. It is not your daughter, it's her illness.
Be patient, she will get her personality back, and be the woman you know and love, but this difficult time has to be lived through first.
Remember at all times, it's no-one's fault - not hers, not yours, and she will get through this, with your love and support.