I'm going to add someone on Facebook and then lie to my girlfriend about it.
Are you for real? Why are you going to hide something like this?
I don't want her to think I'm cheating again.
It's terribly disrespectful to not be open with your romantic partner, and to not tell her the truth about who you're friends with on Facebook. It's better to be honest with her about what you're doing, because that why she has no reason to be suspicious of you.
Nah, I'm going to lie about it and make her think I'm a good guy.
I think it's best to stop this kind of behavior and just be honest with her. Don't you want to have a good relationship, and make it up for her after your cheating? How about talking to her, she may want to help you and save the relationship.