ahh how to?

I'm so sorry this happened to you.
I'm going to say this once and I hope it sinks in- THIS WAS NOT YOUR FAULT!
You were going out with a loser and one of your best friends stabbed you in the back.
I really hope you can stop cutting because you don't need any more pain and any more scars then they have left on you.
You are a beautiful girl who is deserving of the best the male species has to offer (yes, I refer tot hem as a species:P). You need to believe that and move on. One day an amazing guy is going to come along and show you just how special you are.
I have my suspicions that my ex was cheating on me with my very best friend because they seemed to have a better connection then we did but I played dumb and pretended I knew nothing. I'm sorry you were confronted with it.
I'm not very good with advice but I hope this helps somewhat.