If you would lie to someone you love when they tell you they don't mind if you tell the truth, how could you be trusted telling the truth when the admission would be taken poorly?

You've broken her trust, but by telling her and making this post you are being truthful with yourself.

You should seek therapy if not already, and seek out other activities at night to replace what you are saying is a negative usage of a drug. Go to the gym, play video games, knit, do something positive to replace your time smoking and distract your brain.

Have something to show for yourself and for your girlfriend. Write a poem every night and give it to her to show you are dedicated both to changing and to healing your relationship. Even though she didn't even care about the smoking, if you can prove you can change such a habitual thing about self, you can hopefully prove that you can commit to building your trust back with her again.

I'm a random person on the Internet reading this post to be pass time, talk to a therapist.