I lied to my girlfriend to be with her.  Our relationship is built upon lies.
do you want to keep lying about your relationship?
Nothing good comes out of a lie?
You're misleading your partner by faking your feelings. It's gonna hurt them a lot if you continue doing it. It's better you come clean and be honest.
If my lies are harming her, does that mean I'm not a good person if I tell her?
You may hurt her, but the longer that it takes for her to find out the worse it gets when she does.  Honestly will at least let you know whether the relationship can continue.