My best friend is replacing me

Me and my best friend are friends since we were 9. We are now 16. Her name is Sam. When Sam was 11 she meet Ana. Fast forward to high school. We are all going to the same high school, but Ana and Sam are in the same grade. Before, me and Sam would walk to school together every morning and we would walk home everyday after school. But now, Sam walks alone to go get coffe with Ana. At first it was bothering me but I didn't say anything and I got used to it. We even stopped going home together because Sam and Ana were going to get coffe together again. Whenever Sam was in a mood to go for a coffe or just a drink she would call Sam. And if Sam could't go, she would call me. We stopped going out, we stopped going for a walks, because she is always with Sam. I told her about this, she felt sorry and said that things will change. One week after our fight, everything went back to how it was before. I don't know what to do at this point.