She lied, meaning you confronted her about this before saying something along the lines as, "Hey X, do you have drugs or use them?". X told you no (Using X since you didn't give me a name).

Turns out she still does! Now, it comes down to two things.

1. Do you still love her?

2. Is this drug that serious?

Marijuana isn't a big deal honestly, but something like Cocaine or Meth is another story. Take into consideration that she kept this from you because she loved you and was afraid you'd think badly of her and leave her. So back to your question. "How do I get past her lying?" Well, do you want to? Are you okay with this? Knowing she does drugs or sells them can be a big deal when it comes to how bad the drug is.

If you don't like the fact that she does drugs and you feel like you've been betrayed, then I advise leaving so you aren't hurting from this. But that isn't easy, since marriage is on the table. You need to list your pros and cons. She Lied, does or sells drugs, and loves you. You can accept it or not. That's up to you.

What would I do?
Well, if I loved her - but she lied to me. I'd try to find a way around it. Forgive her, but tell her you're hurt and she can't be doing things like that. I can't make a real inference since you didn't tell me how bad this is. If it's weed then fuck it who cares. But anything else, I don't think id want to stick around with that.

Are you serious about her. Yes or No