Basically me and my girlfriend split up and the same night we did she went to a party and one of my 'best mates' kissed her when she was drunk. He told me he was as well but it turns out he wasn't at all AND my friends told him not to try it. But he did anyway. So I when I next saw him which happened to be at school, punched him once to the floor and then when he tried to get up I hit him again.

This is the first time I've ever been violent toward someone else and it didn't feel good at all I thought it would but didn't, and now my ex who I'm trying to sort things out with is so angry at me for what I did because its completely out of character :( I don't condone what I did but I just want to know how to get her to forgive me :(

Someone help me :(

WHY did you two split up in the first place?
Personally I would be VERY pissed off that my ex (who I BROKE UP WITH) got into a fight with someone who I kissed... because it isn't their business.
You aren't together. It isn't your business. You have no right to get violent with someone for kissing someone else (especially when it's not your girlfriend)

Your best bet is to learn from your mistake and move on.

Well firstly getting violent never solves anything so take that as a lesson second the hurt of a friend doing that is never easy you will have to explain to your girl look its up to you what you do were not together I'm sorry for the fight but it hurt to find out a friend would do that I had thought more of him then leave the dust to settle dony push her into getting back together just be honest as to why you were upset your friend knew how you felt about her and done it anyway that's about all you can do good luck

Sounds like you are surrounded by people that do not respect you. Not your girlfriend, and not your friends. Let this ones go and start fresh, that would be my recommendation.

Forget getting her back, you have shown her what you bad side is capable of. Back away guy, and make your amends with this fellow. It will take a lot of time for anyone, let alone this girl, to be anything but afraid of you.