So the ole "its not you, it's me" isn't working..time for, " it's not me, it's you"...
really though, some good advice posted already by other members.
Be firm, honest, respectful and Stay Safe! (she sounds schetchy)
Maybe telling her you do not have romantic and or intimate feelings for her...?
best of luck Bud!
Be kind, don't use it as an opportunity to list all the things you don't like about them, if they lash out stay calm and take it, try to leave it on
good terms if possible. Good luck!