I still smoke but I told my girlfriend I stopped.
Why did you tell your girlfriend that you stopped smoking if you haven't?
She would be disappointed if she knew that I'm still smoking.
Even if she won't like the truth it's wrong to lie to her. Tell her the truth about your smoking.
Why can't I just keep up with my lies?
You shouldn't keep lying because it's not right to deceive people, especially people you care about. I'm sure your girlfriend would be more upset about the lying than she would be about the smoking, so it's best to just tell her.