I recently discovered that my son is addicted to pills (benzos). It opened my eyes to a world I never knew existed. My son is not the person I thought he was. He lied to my face and even lied to himself. In fact, he lied to everyone around him if it meant preventing him from loosing his drugs. His thinking is delusional even when he's sober. The recovery center stated it will take years for him to get his thinking straight even if he stays off the drugs because his emotional growth and logical thinking are stunted and warped. The best way I found to help him is to not help him, not enable him, and to make him suffer the consequences so he can become wiser on a more rapid rate on his own.
I suspect your fiance is not the person you think he is. He's been hiding behind a facade for a long time. Lying has become second nature for him. I recommend you attend nar-anon meetings, hold off on the wedding, and don't get yourself pregnant. Accept the possibility that you may need to leave him. When faced with an ultimatum of you or the drugs, he may pick the drugs. You can't control him. You cannot force him to stop. His decision to stop must be his or otherwise his decision won't be sincere. Don't enable him. Also know that the addiction is progressive, -it's going to get worse if he doesn't choose to stop.
I am sorry you are here. Keep reading.