The original Beatrice's soul was going to die soon, leaving Beatrice's body soulless, but the Goddess (the "author" in our Rae's flashback) sought and assigned a new soul (our Rae) to take over the body and Beatrice's "role". She is the one who spoke to and pushed our Rae off the building, and also granted her wish of getting accepted into a college so that our Rae would agree to taking the "role" of Beatrice.
The original Raeliana coveted the life Beatrice had. The dark priestess informed her that she could take over Beatrice's body when original!Beatrice died if she drank the potion and slept on a specific day/time. This caused our Rae's soul to take over original Raeliana's body instead.
Since original!Raeliana isn't meant to be in Beatrice's body, though, she needed to kill our Rae and her original body to prevent (I assume) her soul from rejecting Beatrice's body and returning to her original body, before time ran out. Hence why she's been trying to kill our Rae all this time.
However, the problem has never been her body or life, as our Rae pointed out. Original Raeliana's personality never allowed her to appreciate her lovely family or circumstance. She blamed her old body and circumstances for her "pitiful" existence, but it's really her own personality that's doomed her to unhappy endings.