hey there Brittany,
Im sorry you feel so lonely right now,
Maybe you could talk to this girl? organise a lunch or something and let her know how your feeling. Let her know that you feel like she is not always there for you when you need her.
Do you have any close friends you could talk to?? They may be able to help with things.
I know feeling lonely is awful but im sure if you just talk to her she will understand
I hope things get better feel free to
pm me if you need some one to talk to.