What should i do with my jealousy towards my friend?

Ok. My friend and i have known this person ever since elementary school and she chooses now to hang out with my best friend because she has almost every cl period with her together. She always sits with us and I get a little jealous because my best friend doesn't really talk to me all that much anymore, and get this, her new friend invited her to go see a cheetah girls concert for her birthday(her friend told me first and then told her. i got really mad but didn't say anything) and my friend excepted and left me hanging! I'm very shy and don't know what to say to her so please help! P.S. Her friend is literally hanging on her after school! like onto her shoulders!