How do I tell my girlfriend that I'm addicted. . .?

Start by telling her how hard it is for you just talking about this and how you really need her to be there for you. Then begin at the beginning. Tell her your history, then move on to the present. Make it clear that you are genuinely sorry for starting up again, and that you really would like to stop but you need her help and support. Just be truthful, and I'm sure she'll be touched with your trust and honesty.