How to say sorry to the girl i love?

okay my girlfriend got into a big fight with me.some kept calling her and i told him to stop peacefully.he kept doing it.so i decided i was going to beat his *** right after skool.i was walking up to him and my girl pulled me away.then he was able to get away.i know kickboxing so i was going to mess him up.i asked my girlfriend why she pulled me away and she said she felt bad for him.later that day i found out that guy was her ex boyfriend.i said some very hurtful things to her after i found out.her friends told me she was crying for like 40 min and the rest of the time she was quiet.after i heard that i really felt bad and started crying myself(dont laugh).but was this a good reason to break up with her?is it possible that she still has feelings for him?did i overreact?they also told me she would be okay with him calling her.if i did overreact how do i say sorry?