Friend trouble? Please help?

Long but please read? OK, so I have two best friends. There are three of us and we would do everything together. We'll call them Selena and Niki. Selena and I were really close but we always included Niki in everything and everything was going great for a couple of months. Then, Selena started getting really buddy-buddy with Niki, and they knew each other longer than I did. And Niki kind of goes with the flow, she's not the type to stand up for herself but Selena is really popular. Then Selena and I started drifting away from each other, our friendship was kind of on-off. And then Selena really started pairing up with Niki. They would sometimes ditch me at recess and they would always be with each other. I started getting kind of annoyed but I never tell those two or they will know I am too needy and Selena is pretty much the only reason I actually have friends. Once she had a fight with me, the whole school took her side and I have no clue why. Anyways, they were overly close to each other and one day a field trip comes up and we were making plans to sit on the bus. They were seats of three and I was like FINALLY! I CAN ACTUALLY BE INCLUDED FOR ONCE! But then, Selena goes and asks another girl to sit with them and I was just like WHAT THE HELL?? but I didn't say anything. She was purposely leaving me out. She knew that she could do whatever she wanted to me and I couldn't do anything because she had a lot more influence and I was just in the school for one year. I don't know who else to be friends with because everybody's all paired up and there are a few girls that would be my friend but they are really gross and annoying and everybody hates them. Ever since Selena decided she was bored of me and started telling everyone I was racist, the whole school turned against me and people would whisper whenever I was near and nobody would sit next to me, I was depressed. I would get nightmares, I dropped 7 pounds, I stopped eating, I cried my eyes dry. Now, when we started getting a little closer, she goes and does this. And I was squished between a boy who picks his nose and a fat teacher. I really want things to go back to the way they were before but she always just ditches me like that and I can't talk to her because she would just tell everyone I was begging to be her friend. What should I do? Please help.