AITAH for breaking up with my ex after she slept with on of her best friend after out "break"

Hello all this my first post so sorry if it isn't proper.

Am I the a hole for breaking up with my ex who had slept whit on of her best friends after our "break" if thats what you want to call it. It was a couple years ago but I still think about it sometimes we were on and off for almost 2 years and I had genuine feeling for her but neither of us were mature at all and I am glad we broke up but im still wondering if im the asshole for that. I am a male(20) she is a female(19) and here best freedom happened to be a female fyi

thank you!