Did you know that I've been cheating on my girlfriend?
You're joking? Why would you cheat on her? How do you feel?!
I feel guilty for doing it.
It is wrong to lie to people about cheating. People are expected to be faithful to their partner in a relationship.
It's okay cause she won't find out.
Even if she never finds out, it is still wrong and eventually the guilt will hurt you way worse than just coming out and admitting to it ever would. While it'll be hard, you need to care about your loved ones and tell them the truth.