I'd take the path of honesty if I were you. You'll feel a lot better mentally when you're not living a double life.
I was dating my girlfriend for about 6 months when I first told her I was a user and she had no idea I was using prior. We moved in together rather quick, but it was then, before she moved in, that I decided to just come clean and tell her I'd been using meth and heroin. At this point, I really wanted to quit anyway, so honesty was the best policy. I just sat her down and said, "I know I told you when we met that this was all in the past. And it honestly was, I started again shortly after we started talking, but this is where I'm at. I'm a user. I use meth and heroin. I relapsed after 7 1/2 years clean and I've been using basically the entire time you've known me." She took it really hard at first, but accepted it and I kept using for a while after. Finally I decided to quit and was doing really good for a couple months, but then I fell back into a cycle with meth that I haven't broken since. She always told me to be honest, but I was so ashamed that I just kept lying about it and she kept catching me.
Well, last night was the final straw. She found my stash, again, and so I fessed up to all the use I've been hiding from her and let it all go. Rather than disposing of my drugs and pipe like the last few times, she just said, "Well, it's clear you've made your choice. You can't be honest with me and I can't keep doing this." She left with all her stuff and our dog. If I wasn't high as a kite at the time, I'd be devastated. I know I will be when I finally come down (I've been smoking on my stash despite hating the shit and wanting to get clean, desperately, for good...). She said she's willing to come back if I get professional help and follow through with it, and so now I'm looking into what all that entails.
But yeah, had I just been honest in the first place and not kept sneaking around behind her back and getting caught, she probably would still be here right now and I wouldn't be stuck alone in my room, high as a kite, waiting for the emotions to set in when I finally come down. But I do feel relieved now that I don't feel so ashamed that I have to sneak around.