I think it sounds like you are putting more into this friendship that you are getting out of it and that really isn't fair. You need to talk to her about it and ask if she really values your friendship, and ask her to be a bit more considerate of your feelings. It completely sounds like she is using you at her convenience and this is not fair on you, you need to either get her to make time for you or treat you as she treats her other friends, or really, just cut her out. Stop talking to her.
She may come running back to you if you do stop talking to her, but that would either give you the choice of giving her one last chance or moving on completely and having friends that treat you as an equal and as though you are worth it. Which you deserve, because you are. :-)
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