1) remind yourself it's NOT your fault that she cheated on you. If she left you, it could have been partly your fault, but cheating ..there's no excuse for that. So don't blame yourself.
2) remind yourself she wasn't who you thought she was, she's not the great girl you thought you were with, if you feel like crying just remember you're crying over someone who is not real, that sweet girl you're crying over doesn't exist, she's an awful person and now she's showed her true colours, try to remember that.
3) please try to ignore her, don't txt her, call her, you deserve better and she doesn't deserve the attention.
4) if she comes asking for forgiveness just remember once a cheater always a cheater, in most cases, so I would suggest don't get back together.
5) BUT most importantly, try not to be too sad, do fun things and keep busy. Remember you didn't lose a great girl, you THOUGHT she was a great girl. It hurts to realize that but you'll be alright with a bit of time and you'll eventually meet a great girl who will love you and respect you

Take gun from my compartment,

go to her house

go through the window

and shoot her in the heart.