Jealousy.. please help?

I'm going into high school next year and I am and for the past while now have been very jealous of one of my best friends. I know her and how she can easily go and befriend people she says she doesn't like which is very annoying. Anyway, 2 very cool people at the highschool really like her. Actually, a lot of people like her and are always trying to hang out with her, never me. It sucks and it makes me really sad. A minute ago a popular girl posted on her wall "oh my gosh we need to hang out this summer! i am soooo glad we'll be at the same school next year" This upsets me a lot more than it should, i've just never been the cool, beautiful favorite. I'm scared that I won't have very many friends and she'll just leave me and be cooler than me and everybody will like her better which is bound to happen, she's also gorgeous and will get all the guys anyway like she does now. It will always be her over me, and I have a miny heart attack whenever I picture the first day of school and her not hanging out with me even though we have been close for so long, it's something she would totally do. I picture everybody she says she doesn't like, all those popular girls saying "come sit heree!! with us!!!" and then she will, and I won't know where to sit or what to do. I know it will happen. I don't want this to ruin my summer, I don't know what to do. please help :'(