[Sigh. Why am I a counselor, I am so fucked up these days.] Okay. First of all, it has to come from you, not from me if it's going to mean anything to her. For whatever reason She loves you.

But here is why she's so upset. Cissie always feels like second best. Always. So when you told her you loved another woman, what she heard was "if this other woman snaps her fingers, Sylar will go to her, because of course she's better than me." So she's hurt and upset because she thinks she's not enough for you.

Maybe you should try to find a way to show her that she is.