Used to lover her, not anymore?

I made a friend beginning of High School year and we were basically inseparable, over the course of the year we began hanging out with other people but we were always together, well this year we are in separate classes and i didn't get to see her, much only during recess. she has gotten closed to one of our mutual friends and seems like now they are now inseparable.I feel is sting in my heart whenever see them do the thinfs that me ana her would usually do. it's like I lost a gem. I feel possessive and I don't like feeling possessive it feels overwhelming mentally in physical draining I really don't want to care but I care just as much. It feels uncomfortable when someone calls us "best friends" because I know she is not anymore,well atleast to me. I dont care whether she feels the same way or not. I just need peace.