How do you handle a person who used to be your best friend, but met another one of your BFFs & changed to much?

My friend (lets call her Sarah) has been a BFF since 3rd Grade! My other friend (lets call her Mandy) has been a BFF since 1st Grade! Mandy was out of school for 2nd and 3rd grade, so Mandy and Sarah never met. Then Mandy came back in 4th grade and I was in class with yet another BFF (lets call her Madison) and another (lets call her Marie.) So I was in class with Madison, who was BFF's with Marie. Mandy only knew me, and same with Sarah. When people meet Madison, they change and become bullies. So thats what happened to Sarah and Mandy. Marie actully stayed nice, and me...I am way to shy and I don't defend myself. Now, Sarah always says %26quot;Go away nobody wants you here.%26quot; So I ask Mandy, and she says %26quot;I don't care if your here%26quot; and Madison, was never mean to me. But Sarah is really mean to me now and I can't take it. I have no other friends!! And I made fun of Marie 1 time and she gets PO at me! When Sarah and Madison dis her like crazy every day. Please tell me how to defend myself, dis her, think of comebacks, make friends with the three people I want to be friends with, Grace, Taylor, and Lucy. Of course, Sarah and Mandy and Marie are their friends, but why not me. I used to be friends with Grace, and I think Lucy wants to be my friend, but idk about Taylor. Please help me! Sorry this is so long.How do you handle a person who used to be your best friend, but met another one of your BFFs %26amp; changed to much?find yourself another best friend.