I cheated on my girlfriend and I don't know what to do.
Are you serious? Did you cheat on her intentionally?
Yes, I did it on purpose but I still feel guilty about it all.
You already know that cheating is wrong and you shouldn't do it if you're feeling guilty, but the only way to take that burden away is to stop being a coward and tell your partner about what you did. It's better to tell even the most painful truths than it is to lie.
I can't tell her, she is going to break up with me.
It sounds like if you cheated on her then you don't deserve her. However, she might forgive you if you are just honest with her.