If it would kill you to be friends with her, then the best thing to do is cut it off. She sounds like she doesn't know what she really wants. She has a boyfriend but was leading you to think that she had feelings--other than platonic ones--for you. Now she says that she just wants to be with her boyfriend. Whether it was intentional or not, she was jerking you around. Of course it won't be easy but you need to move on. Let her know that being just friends is too hard for you to do given your feelings for her.