BiPolar friend problem!!!!?

she isnt REALLY bipolar but OMG!!!!!! She is all happy and playfull then she gets in these like pouty moods and acts all sad and depressed!!! and I ask her wuts wrong and she says NOTHING!!!!! and i'm like o.k??? and then she gets all perkY!!! she also gets pissed when she isn't invited to a sleepover!!! she cant get invited EVERY time!!! i am just so pissed!!!! i just wanna get rid of her!!! she hangs out with the grade above us and acts like i dont exsist then she comes CRAWLING BACK!!!!!! ever since the day i met her!!( which i regret!!!) i know this sounds AWFUL and horrible but it is SOOOOOOO true and I just need to know how to handle it!! and dont you dare say just go along with it cause i have for to long now!!!!!!!!!!!!!!! she is just so pissy!!!!