My relationships are alywas short, turbulent, and end badly. So I'm not sure if this is advice or a firm "don't do what I did"
I never allow anyone to have any say over my drug use. It's a psychological issue from my past, it reminds me of my mother and I just cannot handle that in a girlfriend. I know, issues
However, it seems he has asserted control over YOUR use, with ketamine, so that in my mind gives you every right to be as angry with him as he would be with you had you violated that trust. Not only that, but actually lying to you is obviously totally out of order, even though I probably would have prolonged a lot of my relationships if I didn't have a huge problem with telling untruths to my significant other. Still, mistrust is poison to a relationship, absolute poison, and he should have realized the grave implications of his actions before he decided to violate your agreement and then lie to cover it up.
That being said, I have lied before to cover my addiction, and to this day I have problems thinking of it as lies. To me it's still a different category, it was NECESSITY, I had to get high and I knew I couldn't if I was honest about it. I'm getting the feeling you don't know the full extent of your fiance's drug problem. I have personally hid very serious drug abuse from people I was living with, and have known others doing even more drugs with even more clueless partners. In most cases they are not experienced with drugs, while you are, so maybe it's not the same.
However, you'd be surprised how easy it is to cover up a drug problem without actually lying much, and it's even easier to convince yourself that drugs are so necessary that anything you say to stay on them is justified. I know in my head that lying about drug use is lying, but I still can't totally associate it with lying about other things, which I despise and never do.
Anyway, I'm rambling now, but I wish you the best of luck with this situation. Whatever you do, don't be rash. You obviously love this man, and have trusted him untill now. One incident, especially related to a weakness with drugs, should not necessarily cancel out years of feelings and history.
You're a great person, and I feel for you going through this. PLUR, comrade
--- G.