Why are they so rude?

About a month or so ago, my friends started to run away from me or be really rude to me. I gave them everything, even played a game I didn’t like once in a while because of them. I gave them birthday presents, but they didn’t even say ‘Happy Birthday!’ when it was mine. We have been friends since the start of school, but now they ditched me for new friends with the popular group, with my enemy, Ella. They shared all my secrets with her. I got new friends since then, but they are still SO mean. Why did they ditch me? Why are they so rude to me? All I ever did was be rude to Ella, but that’s because I respect others the same way they respect me. I feel as if they are always making fun of me, and I HAVE seen and heard it. Ella has been so mean I have actually said really bad things to her and defended myself from her, since I am about 6ft. and she’s around 4ft. Why are they like this? Can I stop it? They’ve been mentally annoying me.